Grilled Flat Iron Steak With Creamy Mushroom Sauce

Grilled Flat Iron steak pairs well with the umami flavors of the creamy mushroom sauce.

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Flat iron steaks are one of the most underrated cuts of beef on the market today.

When grilled correctly, you’re left with juicy, tender steak that’s packed full of flavor. 

In this recipe, I’ll show you my favorite method for grilled flat iron steak. The creamy mushroom sauce will make something that already tastes great even better.

Grilled Flat Iron Steak With Creamy Mushroom Sauce #Shorts

What is a Flat Iron steak?

Have you ever tried a Flat Iron steak? 

It’s not the most popular steak cut and tends to be overshadowed by the more popular Filet Mignon, but don’t let its lack of popularity fool you. The Flat Iron steak is a quality cut and rivals even the highest quality Filet Mignon when it comes to tenderness.

The Flat Iron (commonly shortened to flatiron) steak comes from the primal chuck, which is cut from the shoulder and provides beef known for its rich beefy flavor, tenderness, and good marbling.  Other names include “Oyster Blade Steak”, “Butler Steak” and “Top Chuck Steak”.

The flat iron tends to be uniform in thickness and rectangular in shape. Because of its muscular structure, it benefits from being cooked to a perfect medium rare. Anything past medium (145°F) and the steak will start drying out quickly.

Reverse searing: Worth the hype?

The traditional culinary technique of searing generally starts with a sear over direct heat, then finishes the cut of meat over indirect heat. 

Recently, reverse searing has gained a lot of popularity in the culinary world. The benefit of a reverse sear is that you can still achieve a flavorful crust on your steak but maintain a more evenly cooked internal temperature throughout the rest of the steak.

grilled flat iron steak on wire rack over coals
Reverse sear works best for thicker cuts of meat

A traditional sear tends to leave a band of well-done meat around the outside of the steak because it is seared at a high temperature first and then continues to cook over indirect heat after the sear. 

Both methods can leave you with an amazing flatiron steak, but the reverse sear method will give you better texture and a much more even cook on the interior of the steak.

I prefer to reverse sear for any pieces of meat thicker than 1.5 inches.

How to grill flatiron steak

1. Season with a simple SPG rub

Some people like to get fancy with their steak seasoning blends and marinades, but I’m a firm believer that simplicity is key when you are working with quality beef. 

For this recipe, I removed the Flat Iron from the refrigerator and let it rest for 1 hour to come closer to room temperature.

I seasoned the flat iron steak with the Kosmos Q SPG Rub. It is a special blend of sea salt, black pepper, and garlic with just a touch of sugar and spices to really boost the flavor of steak, chicken, and pork.

raw seasoned flat iron steak on chopping board
If you don’t have a rub on hand, a simple sprinkle of salt and pepper still does it justice.

2. Fire up the grill for two-zone cooking

There is nothing quite like a great steak cooked over charcoal, so we used the versatile Nomad Grill and set it up for two-zone cooking.

Two-zone cooking is a method where you have one side of the grill that is producing high-temperature, direct heat and the other side is producing no heat so the meat can cook indirectly. 

Essentially, we pushed the charcoal off to one side of the grill and let the steak start cooking on the other side. 

If you’re cooking on a gas grill with multi burners you can leave a burner off to create the same effect.

raw flat iron steak on the grill

We smoked it at about 250°F on the indirect side until the steak reached an internal temperature of 110°F.

At that point, we moved the steak to the direct heat side, right above the charcoal, and gave it a good reverse-sear until it hit 130°F internal.

3. Patience is a virtue: It’s time to rest

Arguably one of the most important steps in cooking a perfect steak is the resting period after cooking. If you cut into your steak immediately after you pull it off the grill you are going to end up losing a lot of the flavor that you just worked so hard to achieve.

When meat cooks, the muscle fibers in the meat contract and push the water in the steak to the outside. The resting period allows the meat to cool slightly, and the contracted muscle fibers will begin to relax as the temperature comes down. 

Additionally, there will be less pressure in the center of the steak from the contracted muscle fibers and the moisture in the meat will redistribute evenly, thus locking in all that juice and flavor that makes a good steak great.

The resting period varies depending on the size of your steak and for this Flat Iron, we let it rest for about 10 minutes. 

We like to tent steaks with a bit of foil to maintain as much heat as possible and the result is a perfectly cooked, juicy steak that is packed full of flavor.  

Making the creamy mushroom sauce

While your steak is resting, it’s time to make the creamy mushroom sauce. You can serve this steak on its own, but I find the mushroom sauce pairs beautifully.

sliced onions cooking in a cast iron skillet on a grill

Place a cast-iron skillet over the hot coals on your grill and add a drizzle of olive oil. Place your sliced onions in the pan and add a touch of salt. Let the onions cook for about 3 minutes or until they are translucent. 

Then add the mushrooms to the same pan and sauté until they are tender about 5 minutes.

sliced onions and mushrooms in a skillet on the grill
Mushrooms tend to cook down a lot, so add more if love them.

Once your mushrooms and onions are both soft and tender, you can add the heavy cream, Dijon mustard, Worcestershire sauce, grated parmesan, and cracked black pepper.

Give the sauce a good mix and allow it to cook for about 5 minutes more.

mushroom sauce in a cast iron skillet on the grill
It’s a super simple sauce that packs a ton of flavor and pairs perfectly with a perfectly cooked steak. 

The great part about the sauce is that it is extremely versatile, and you can add more of whatever you like and really make it your own! It also tastes amazing when served over asparagus or broccoli.

Serving your flat iron steak

Once your steak has rested and your sauce is ready, cut the meat into ¼” slices and lay it on a plate. Pour the mushroom cream sauce over the top and this steak is ready to enjoy! 

sliced flat iron steak with mushroom sauce on top on a wooden chopping board
The umami flavors of the creamy mushroom sauce provide a great accompaniment for this beautiful steak.

Other steak recipes to try

Sides for grilled steak

Grilled Flat Iron Steak Recipe

grilled flat iron steak sliced

Grilled Flat Iron Steak

Reverse seared Flat Iron steak served with a creamy mushroom sauce
5 from 2 votes
Print Pin Rate
Course: Main Course
Cuisine: American
Prep Time: 5 minutes
Cook Time: 24 minutes
Pre cook resting time: 1 hour
Total Time: 1 hour 29 minutes
Servings: 2 people
Calories: 1201kcal
Author: Jordan Hanger


  • 12 oz Flat Iron steak
  • 1 tbsp BBQ rub

Creamy mushroom sauce (optional)

  • ½ cup white onions diced
  • 1 cup mushrooms diced
  • ½ cup heavy cream
  • 1 tbsp Worcestershire sauce
  • 1 tbsp Dijon mustard
  • ¼ cup Parmesan cheese grated
  • 1 tsp black pepper ground


  • Remove the steak from the refrigerator and allow to rest at room temperature for 1 hour.
  • Pre-heat your grill to 250°F and set it up for two-zone cooking (details above).
  • When the steak has rested, season it on both sides with BBQ rub.
  • Place the steak on the grill over indirect heat and cook for about 20 minutes, or until the internal temperature reaches 110°F.
  • Transfer the steak to the direct-heat side of the grill and sear for 2 minutes per side.
  • Remove steak from the grill, tent with foil and rest for 10 minutes.
  • After your steak has rested, cut into ¼” slices and place them on your serving plate.
  • Pour the mushroom cream sauce over the top of the steak and serve immediately.

Creamy mushroom sauce

  • While your steak is resting, place a cast iron pan over the coals on your grill.
  • Once the pan is hot, add the olive oil and onions. Sauté for about 3 minutes until the onions are translucent.
  • Add the mushrooms to the pan and sauté for 5 minutes, or until the mushrooms are tender.
  • Add the heavy cream, Worcestershire sauce, Dijon mustard, Parmesan cheese, and cracked black pepper to the pan.
  • Stir for about 5 minutes and allow the sauce to form.


Grilled Flat Iron Steak With Creamy Mushroom Sauce #Shorts


Calories: 1201kcal | Carbohydrates: 23g | Protein: 81g | Fat: 88g | Saturated Fat: 45g | Trans Fat: 1g | Cholesterol: 405mg | Sodium: 1044mg | Potassium: 1796mg | Fiber: 4g | Sugar: 8g | Vitamin A: 2086IU | Vitamin C: 12mg | Calcium: 491mg | Iron: 12mg
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