Preheat the oven to 400°F (204°C). Line a rimmed baking sheet with parchment.
Combine the ground beef, egg, breadcrumbs, Parmesan, Italian seasoning, garlic powder, salt, and pepper in a bowl. Mix gently until evenly combined.
Shape the mixture into approximately 16 small, evenly sized meatballs and arrange them on the prepared baking sheet.
Bake for approximately 15–18 minutes, or until the ground beef reaches an internal temperature of 160°F (71°C).
Cut the French bread lengthwise and divide it into four equal portions. Place cut-side up on a baking sheet and toast for 3–4 minutes.
Spread marinara sauce evenly across each bread portion, using a moderate layer to prevent sogginess.
Distribute the cooked meatballs among the four portions, then top evenly with mozzarella and optional Parmesan.
Return to the oven for 8–10 minutes, until the cheese has melted and the toppings are hot. Watch closely if using the broiler for extra browning.
Rest for 2–3 minutes, garnish with fresh basil or parsley if desired, and serve warm.