Prepare a grill fire to about 300° with pecan or hickory for smoke flavor.
Mix ground beef and bbq sauce together and form into about 1 1/2 - 2 inch meatballs. A medium cookie scoop works perfect.
Place meatballs on a grill pan and cook over indirect heat with the grill lid closed.
Cook meatballs about 15- 20 minutes or until internal temperature reaches 165° internal temperature using a meat thermometer.
Remove from grill and set aside.
Make marinara sauce by adding tomato sauce, fennel seeds, garlic, salt, basil, oregano, thyme and brown sugar to a small saucepan. Cook over medium heat about 5 minutes or until heated through. Keep warm while you toast the sandwiches.
Slice hoagie buns and place bottoms on a baking sheet.
Top with meatballs, then shredded mozzarella.
Place meatball subs and top buns over indirect heat on the grill and close the lid.
Cook about 5- 8 minutes or until cheese is melted and top buns are toasted.
Remove from grill.
Top with marinara sauce and serve immediately.