01 - Set 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, and pepper; mix just until blended.
03 - Form the mixture into 16 meatballs, approximately 1 1/2 inches in diameter, and arrange them on the prepared baking sheet.
04 - Bake for 15 minutes until lightly browned and fully cooked.
05 - Heat olive oil in a skillet over medium heat; sauté onion for 2-3 minutes until soft, add garlic and cook for 1 more minute.
06 - Add marinara sauce to skillet, bring to a gentle simmer, then add baked meatballs and coat them well; simmer uncovered for 10 minutes.
07 - Slice sub rolls lengthwise without cutting through and toast in the oven for 2-3 minutes until lightly crisp.
08 - Divide meatballs and sauce evenly among toasted rolls, then top each with shredded mozzarella and a sprinkle of parmesan.
09 - Place assembled subs on a baking tray and bake for 3-5 minutes until cheese is melted and bubbly.
10 - Optionally sprinkle fresh basil or parsley over subs and serve hot.