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 just combined.
03 - Form mixture into 12-16 meatballs, about 1 1/2 inches in diameter. Arrange on prepared baking sheet and bake for 18-20 minutes until cooked through and browned.
04 - Heat olive oil in a large skillet over medium heat. Cook onion until soft, about 3 minutes. Add garlic and red pepper flakes; cook for 30 seconds.
05 - Add marinara sauce and dried basil to the skillet. Simmer for 5 minutes.
06 - Transfer baked meatballs into the sauce. Simmer together for 5-7 minutes, spooning sauce over meatballs to coat thoroughly.
07 - Slice sub rolls without cutting completely through. Place 3-4 meatballs with sauce in each roll.
08 - Top meatballs with shredded mozzarella and sprinkle with Parmesan cheese.
09 - Arrange subs on a baking sheet and broil on high for 2-3 minutes until cheese is melted and bubbly.
10 - Garnish with fresh basil or parsley if desired, and serve warm.