Warm the olive oil in a large cast iron skillet (10 to 12 inches in diameter) over medium-low heat. Add the diced onion and cook until softened, stirring occasionally.

Incorporate the crushed garlic and continue to sauté until the onions are translucent and aromatic.

Stir in the cooked meatballs along with the marinara sauce and Italian seasoning. Allow the mixture to simmer gently on medium-low heat for approximately 20 minutes.

Meanwhile, bring a large pot of water to a boil and cook the spaghetti noodles slightly under the recommended time (about 2 minutes less than package directions). Reserve some of the pasta water before draining.

Using a slotted spoon, carefully lift the meatballs out of the sauce and set them aside temporarily.

Transfer the cooked pasta directly into the skillet containing the sauce, using a slotted spoon. Add about ½ cup of the reserved pasta water and stir gently to ensure the spaghetti is evenly coated in the sauce.

Return the meatballs to the skillet, arranging them over the spaghetti mixture.

Generously sprinkle the surface with the shredded cheese blend and grated Parmesan.

Place the skillet in a preheated oven at 350°F and bake for 20 to 30 minutes, or until the cheese is fully melted and the dish is bubbling hot throughout.