Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

In a large mixing bowl, combine ground beef, breadcrumbs, and optional grated onion or zucchini. Season with salt, pepper, and other seasonings.

Form the meat mixture into golf ball-sized meatballs, enclosing a piece of mozzarella cheese in the center.

Place the meatballs on the baking sheet and bake for 25-30 minutes until golden brown and cooked through.

While the meatballs bake, cook spaghetti in boiling salted water until al dente, about 8-10 minutes. Drain and return to the pot with olive oil.

Heat marinara sauce in a saucepan over medium heat, stirring occasionally for 5-7 minutes.

To serve, place spaghetti on plates, top with meatballs, and drizzle marinara sauce over everything. Garnish with parsley.