Preheat your oven to 375F and move the rack to the top third of the oven.
Add the oil and onion to a skillet over medium-high heat and sauté for 4-5 minutes.
Stir in the beef and garlic and continue cooking, breaking the meat up with your spoon as you go along, until the beef has browned (about 8-10 minutes). Spoon out excess fat if there's a lot, and then transfer the cooked beef to a 9x13 casserole dish.
Add the marinara sauce, cheese tortellini, and some salt & pepper to the casserole dish, then toss it all together and spread it out in an even layer.
Sprinkle the cheeses over top, then cover the dish tightly with foil.
Bake for 35-40 minutes or until the tortellini is cooked and the cheese has melted. Optional: take the foil off once it's done, then broil for a few minutes to brown the cheese up a bit.
Notes
Serves 6-8 depending on how much people eat/what else it's served with.
Since the tortellini isn't boiled, it's important that you use refrigerated tortellini since it cooks fast. If using the dried kind, boil it according to package directions first.
I used a bagged mozza/cheddar blend (2 cups of it).
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.