Preheat the oven to 375°F.
Heat a frying pan over medium high heat. Once hot, add 1 Tbsp olive oil, 2 minced garlic cloves and ½ cup chopped onion. Cook until onion turns translucent, about 1-2 minutes.
Add 1 lb lean ground beef and break apart using the tip of a wood spatula. Cook until ground beef turns brown and is no longer pink. Use a folded paper towel to swish around with spatula and soak up any excess grease.
Now add 1 cup sliced cherry tomatoes and cook for 1 minute to soften. Remove the pan from heat and scoop into a large bowl.
In a large bowl, add the cooked ground beef mixture, 18 oz uncooked tortellini pasta, and 1 cup of mozzarella cheese. Mix together.
Next, add the 21 oz jar of marinara sauce on top and mix everything together.
Pour the mixture into a baking dish and spread into an even layer using a spatula. Sprinkle 1 cup of shredded mozzarella and ¼ cup parmesan cheese on top.
Cover the casserole dish with foil and bake for 30 minutes. Then remove the foil and broil an additional 1-2 minutes.
