Fire up your smoker to 250°F.
Cut the beef and pork into 1" cubes. Grind as per the instructions of your meat grinder.
Place the ground meat into a bowl, and add the eggs, fennel seeds, seasoning, garlic powder, and breadcrumbs. Mix everything together until fully incorporated.
Form the mixture into 2-3 oz meatballs, roughly the size of golf balls, and place into a cast iron pan.
Put the pan into the smoker, close the lid, and let them smoke until they have reached an internal temperature of 140°F.
Tip the marinara sauce into the pan, ensuring all the meatballs are covered, and let them smoke for another 30 minutes, then remove from the smoker.
Toast the subs in your oven. Stuff with as many meatballs as you can, spoon on some marinara sauce, and top with mozzarella cheese. Garnish with a little parsley and serve with extra sauce on the side.