01 - Set oven to 400°F and line a baking sheet with parchment paper.
02 - In a large bowl, combine ground turkey, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, pepper, and milk; mix gently until just combined.
03 - Shape mixture into sixteen 1.5-inch meatballs and place evenly spaced on the prepared baking sheet.
04 - Bake meatballs for 15 to 18 minutes until golden and cooked through.
05 - While meatballs bake, heat olive oil in a large skillet over medium heat; cook chopped onion for 3 to 4 minutes until softened, then add minced garlic and cook 1 minute longer.
06 - Add crushed tomatoes, dried basil, oregano, sugar, salt, and pepper to skillet; simmer for 8 to 10 minutes, stirring occasionally.
07 - Transfer baked meatballs to the sauce; simmer an additional 5 to 7 minutes, spooning sauce over meatballs to coat thoroughly.
08 - Preheat broiler. Slice sub rolls and place on baking sheet; broil for 1 to 2 minutes until lightly toasted.
09 - Place four meatballs with sauce on each roll; top with shredded mozzarella and a sprinkle of Parmesan cheese.
10 - Return assembled subs to broiler for 2 to 3 minutes until cheese is melted and bubbly.
11 - Optionally garnish with fresh basil leaves and serve immediately.