Cook pasta for 1 minute less than package directions for al dente pasta.
Meanwhile, heat a large oven-proof skillet over medium heat, and add in olive oil.
Stir in onion and garlic. Cook for about 4-5 minutes, stirring regularly.
Add in marinara sauce, cooked meatballs, and cooked pasta. Stir to combine.
Top with slices of mozzarella.
Bake for about 10 minutes, or until cheese has melted and pasta is hot and bubbly.
Notes
Pasta: I like to use a tubular pasta like rigatoni, penne, or ziti in this recipe.
Pasta sauce: I like to cook onions and garlic to add to my sauce because it adds a nice layer of extra flavor to the pasta. If you have homemade sauce, feel free to sub it for the jarred kind.
Meatballs: I use homemade meatballs if I have them. Otherwise, frozen meatballs (that have been cooked) will work just fine.
Mozzarella: You can use shredded mozzarella cheese if you prefer.