In a large cast iron skillet, sauté the finely diced onions in olive oil over medium to low heat. Continue by adding the crushed garlic and sautéing until the onions attain a tender texture.

Introduce the fully cooked meatballs to the skillet.

Subsequently, pour in the marinara sauce and the teaspoon of Italian seasoning. Allow the mixture to simmer over medium to low heat for a duration of approximately 20-30 minutes.

Concurrently, in a separate large pot, prepare the spaghetti noodles in accordance with the manufacturer's instructions, ensuring they are undercooked by a margin of 2 minutes.

Employing a large slotted spoon, delicately remove the meatballs from the sauce and place them aside momentarily.

Transfer the cooked spaghetti into the skillet containing the marinara sauce. Incorporate approximately 1/2 cup of the pasta cooking water. Mix gently, ensuring the pasta is uniformly coated with the sauce.

Restore the cooked meatballs to their position atop the spaghetti.

Generously sprinkle the top with both the grated cheese and the finely grated parmesan.

Commence baking at 350°F for an estimated 20-30 minutes or until the dish exhibits a bubbly appearance and the cheese is thoroughly melted.