01 - Preheat the 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 arrange evenly on the prepared baking sheet.
04 - Bake meatballs for 15 to 18 minutes until golden and fully cooked through.
05 - While meatballs bake, heat olive oil in a large skillet over medium heat; sauté onion 3 to 4 minutes until softened, then add garlic and cook 1 more minute.
06 - Add crushed tomatoes, basil, oregano, sugar, salt, and pepper; simmer gently for 8 to 10 minutes, stirring occasionally.
07 - Add baked meatballs to the sauce, simmer an additional 5 to 7 minutes, spooning sauce over meatballs.
08 - Preheat broiler. Split sub rolls and place on a baking sheet; toast under broiler 1 to 2 minutes until lightly browned.
09 - Place four meatballs with sauce on each roll; top with shredded mozzarella and sprinkle with Parmesan cheese.
10 - Broil assembled subs for 2 to 3 minutes until cheese is melted and bubbling.
11 - Garnish with fresh basil if desired and serve immediately while hot.