01 - Preheat oven to 400°F and line a baking sheet with parchment paper.
02 - In a large bowl, combine ground turkey, breadcrumbs, Parmesan, egg, minced garlic, parsley, oregano, salt, and pepper. Mix gently until just combined.
03 - Shape mixture into 16 uniform meatballs, approximately 1 1/2 inches in diameter, and arrange on prepared baking sheet.
04 - Bake meatballs for 15 to 18 minutes until fully cooked and lightly browned on the outside.
05 - In a saucepan over medium heat, warm olive oil. Sauté onion for 3 to 4 minutes until softened, add garlic and cook for 1 more minute.
06 - Stir in marinara sauce, dried basil, and red pepper flakes if using. Simmer gently for 10 minutes, stirring occasionally.
07 - Add baked meatballs to the sauce and simmer an additional 5 minutes to blend flavors.
08 - Slice sub rolls horizontally. Spread the cut sides with softened butter and toast under a broiler or in a skillet until golden brown.
09 - Place four meatballs and sauce into each toasted roll. Sprinkle evenly with shredded mozzarella cheese.
10 - Place assembled subs under the broiler for 2 to 3 minutes until the cheese melts and bubbles.
11 - Optionally garnish with fresh basil or parsley. Serve immediately while hot.