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, garlic, parsley, oregano, salt, pepper, and milk. Mix until just combined without overmixing.
03 - Shape mixture into 16 meatballs approximately 1.5 inches in diameter and arrange on the prepared baking sheet. Bake for 15 to 18 minutes until golden and cooked through.
04 - While meatballs bake, heat olive oil in a large skillet over medium heat. Sauté onion for 3 to 4 minutes until softened, add garlic and cook for 1 minute.
05 - Add crushed tomatoes, dried basil, oregano, sugar, salt and pepper to the skillet. Simmer gently for 8 to 10 minutes, stirring occasionally.
06 - Add baked meatballs to the sauce and simmer for an additional 5 to 7 minutes, spooning sauce over the meatballs to coat evenly.
07 - Preheat broiler. Slice sub rolls and place on a baking sheet. Toast under broiler for 1 to 2 minutes until lightly browned.
08 - Place 4 meatballs with sauce on each toasted roll. Top with shredded mozzarella and sprinkle with Parmesan cheese.
09 - Broil assembled subs for 2 to 3 minutes until cheese is melted and bubbly.
10 - Optionally garnish with fresh basil leaves and serve immediately while hot.