Sautéing Aromatics: Begin by sautéing the diced yellow onion in olive oil over a medium-low flame. Once the onions start to soften, add the crushed garlic and continue to sauté until the onions are tender.

Adding Meatballs: Introduce the fully cooked meatballs to the skillet, allowing them to warm through and absorb the flavors of the aromatics.

Simmering with Sauce: Add the San Marzano marinara sauce and Italian seasoning to the skillet. Reduce the heat and allow the mixture to simmer for approximately 20-30 minutes, permitting the flavors to meld together.

Cooking Spaghetti: Concurrently, prepare the spaghetti noodles as per the manufacturer’s instructions in a separate pot, ensuring that they are slightly undercooked by about two minutes.

Combining Ingredients: Using a large slotted spoon, carefully remove the meatballs from the sauce and set them aside. Transfer the partially cooked spaghetti into the skillet, incorporating about 1/2 cup of the pasta cooking water. Gently mix to ensure the spaghetti is evenly coated with the sauce.

Assembling the Dish: Return the meatballs to the skillet, placing them atop the spaghetti. Evenly sprinkle the grated cheese and parmesan over the dish.

Baking: Preheat the oven to 350°F (175°C). Bake the assembled dish for 20-30 minutes, or until it is bubbling and the cheese has melted to a golden brown. Serve hot.