01 - Heat 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 for 15 to 18 minutes until golden brown and fully cooked through.
05 - While meatballs bake, heat olive oil in a large skillet over medium heat. Cook onion until softened, about 3 to 4 minutes, then add garlic and cook for 1 minute.
06 - Stir in crushed tomatoes, basil, oregano, sugar, salt, and pepper. Simmer for 8 to 10 minutes, stirring occasionally.
07 - Add baked meatballs to the sauce and simmer an additional 5 to 7 minutes, spooning sauce over meatballs.
08 - Preheat broiler. Split sub rolls and toast on a baking sheet under broiler for 1 to 2 minutes until lightly toasted.
09 - Place four meatballs with sauce onto each roll. Top with shredded mozzarella and a sprinkle of Parmesan cheese.
10 - Broil assembled subs for 2 to 3 minutes until cheese is melted and bubbly.
11 - Scatter fresh basil over sandwiches if desired. Serve immediately while hot.