01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, combine ground turkey, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, and pepper. Mix until just combined, being careful not to overwork the mixture.
03 - Shape mixture into 16 to 18 small meatballs and place on the prepared baking sheet in a single layer.
04 - Bake meatballs for 15 to 18 minutes, until golden brown and cooked through.
05 - While meatballs bake, heat olive oil in a large skillet over medium heat. Sauté chopped onion until softened, approximately 3 minutes.
06 - Pour marinara sauce into the skillet and bring to a gentle simmer over medium heat.
07 - Transfer baked meatballs to the simmering sauce, stir gently to coat, cover with a lid, and simmer for 10 minutes.
08 - Cut sub rolls lengthwise without cutting completely through. Toast under the broiler until lightly crisp, watching carefully to prevent burning.
09 - Arrange 4 to 5 meatballs with sauce in each toasted roll. Top generously with shredded mozzarella and a sprinkle of Parmesan cheese if desired.
10 - Place assembled subs on a baking tray and broil for 2 to 3 minutes, until cheese is completely melted and bubbly.
11 - Remove from broiler and garnish with fresh basil if desired. Serve immediately while hot.