01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - Combine ground turkey, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, black pepper, and milk in a large bowl. Mix gently until just combined.
03 - Shape the mixture into sixteen 1.5-inch meatballs and arrange them evenly on the prepared baking sheet. Bake for 15 to 18 minutes until golden and cooked through.
04 - In a large skillet, heat olive oil over medium heat. Sauté chopped onion for 3 to 4 minutes until softened, then add minced garlic and cook for 1 minute.
05 - Add crushed tomatoes, dried basil, oregano, sugar, salt, and pepper to the skillet. Simmer for 8 to 10 minutes, stirring occasionally.
06 - Add the baked meatballs into the tomato sauce and simmer together for 5 to 7 minutes, spooning sauce over the meatballs.
07 - Preheat the broiler. Slice the sub rolls and place them on a baking sheet. Toast under the broiler for 1 to 2 minutes until lightly browned.
08 - Divide meatballs and sauce evenly among the rolls. Top each with shredded mozzarella and sprinkle with Parmesan cheese.
09 - Broil the assembled subs for 2 to 3 minutes until the cheese is melted and bubbly.
10 - Optionally garnish with fresh basil leaves. Serve immediately while hot.