Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper. In a large mixing bowl, combine ground meat, breadcrumbs, egg, grated onion, minced garlic, Italian seasoning, salt, and pepper. Mix gently with your hands until just combined, about 30 seconds.

Roll mixture into meatballs about 1 to 1.5 inch in diameter (approximately 16 to 20 meatballs). Place on prepared baking sheet with space between each. Bake for 18 to 20 minutes until browned and they reach 160°F internal temperature on a meat thermometer.

While meatballs bake, warm marinara sauce in a large saucepan over medium heat. Once meatballs are done, transfer them to the simmering sauce and let simmer together for 10 to 15 minutes.

Split sub rolls open without cutting all the way through. Optional: brush insides with butter and toast in oven for 3 to 5 minutes until lightly crisp.

Spoon 4 to 5 meatballs with sauce into each roll. Top generously with mozzarella and provolone cheese. Place under broiler for 2 to 3 minutes, watching carefully, until cheese is melted and bubbly. Sprinkle with parmesan and fresh herbs if desired. Serve immediately.