For the meatballs, you can use cooked and frozen from a store or homemade fresh cooked or cooked and frozen meatballs. Chicken, beef, or pork, up to you. I have a shortcut for basic chicken meatballs listed in the Recipe Notes or you can make the meatballs with any recipe you have.
In a large oven proof skillet, or a large regular skillet if you are going to use a baking dish, heat the olive oil over medium heat and cook the onion until soft, then add the garlic and cook for another 1-2 minutes, then add the meatballs, pasta sauce, and the Italian seasoning. Simmer for 20-30 minutes to warm the meatballs through.
While the meatballs are warming up, bring a pot of water to a boil and cook the spaghetti until just tender then drain, retaining the water from the pot.
With a slotted spoon remove the meatballs from the sauce and set aside.
Add the spaghetti to the pasta sauce, add in 1/2 cup of water from the pasta and gently stir to coat the spaghetti. Spread the spaghetti evenly in the skillet then add the meatballs on top.
If you are going to be using a baking dish (or two) pour the spaghetti into the baking dish and spread it evenly in the dish, then add the meatballs on top.
Sprinkle the cheeses on top of the meatballs and bake for 20-30 minutes or until hot and bubbly and the cheese is melted. Serve with a salad and bread.