Preheat the oven to 375F and move the rack to the top third of the oven.
Cook the noodles for a minute or two less than the package instructions suggest, then drain them and transfer to a 9x13 casserole dish.
Meanwhile, add the olive oil and onion to a skillet over medium-high heat. Cook for 4-5 minutes.
Stir in the garlic and cook for about 30 seconds.
Add in the marinara sauce and meatballs and heat through. Important: if using frozen meatballs, it's important to cook them until thawed (depending on the size/type of meatballs, this may take 8-10+ minutes). Simmer over medium heat (or lower if it's bubbling too much) and stir occasionally. Break a meatball in half to ensure that it's fully thawed.
Pour the skillet contents into the casserole dish and add half the cheese and toss together (I also add some salt & pepper at this point), then spread it out evenly.
Top the casserole with the remaining cheese and bake, uncovered, for 10-15 minutes or until it's hot and bubbly. Optional: broil it for a few minutes (watch it so it doesn't burn) to brown the cheese.
For convenience sake, you can use pre-made frozen meatballs. Use as many/few as you like. 24 oz. of them made quite a lot of casserole, so you could even use half the amount if you're only feeding 4 people, for example. Thawed meatballs or fresh homemade meatballs (my recipe is linked) will work too (any kind of meatballs will be fine).
Nutritional information is provided as a courtesy only and should be construed as an estimate rather than a guarantee. Ingredients can vary and Salt & Lavender makes no guarantees to the accuracy of this information.