- Start by preheating your oven to 375 degrees Fahrenheit and position the oven rack in the upper third of the oven.

- Take a skillet and heat the oil over medium-high heat. Add the onion and sauté for about 4-5 minutes.

- Stir in the beef and garlic, and continue to cook, breaking up the meat with your spoon, until the beef has browned, which should take around 8-10 minutes. If there's excess fat, spoon it out, and then transfer the cooked beef to a 9x13 casserole dish.

- Into the casserole dish, add the marinara sauce, cheese-filled tortellini, and a pinch of salt and pepper. Mix everything together and spread it out evenly.

- Sprinkle the cheeses on top, and then cover the dish securely with foil.

- Bake for 35-40 minutes or until the tortellini is fully cooked, and the cheese has melted. Optionally, you can remove the foil once it's done and broil for a few minutes to give the cheese a nice golden brown finish.