01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Combine ground turkey, breadcrumbs, Parmesan, egg, garlic, parsley, oregano, salt, and pepper in a large bowl. Mix gently until combined without overmixing.
03 - Shape mixture into 12-16 meatballs about 1 1/2 inches in diameter and arrange on the prepared baking sheet.
04 - Bake meatballs for 18-20 minutes until cooked through and lightly browned.
05 - Heat olive oil in a large skillet over medium heat. Sauté onion until soft, about 3 minutes. Add garlic and red pepper flakes and cook for 30 seconds.
06 - Add marinara sauce and basil to the skillet. Simmer gently for 5 minutes.
07 - Transfer baked meatballs into the skillet with sauce. Simmer together for 5-7 minutes, spooning sauce over meatballs to coat thoroughly.
08 - Slice sub rolls without cutting all the way through. Place 3-4 meatballs with sauce inside each roll.
09 - Top each sub with shredded mozzarella and sprinkle with grated Parmesan cheese.
10 - Place subs on a baking sheet and broil on high for 2-3 minutes until cheese is melted and bubbly.
11 - Garnish with fresh basil or parsley if desired and serve warm.