Preheat oven to 400 degrees and bring a pot of water to boil for the tortellini. Cook the tortellini according to package instructions while preparing the rest.
Meanwhile, in a large, oven-proof skillet, heat the olive oil over medium-high heat. Add the onion and cook until soft, about 2-3 minutes.
Add the garlic and cook until fragrant, about 1 minute.
Add the Italian Sausage and break it up as it cooks. Cook until the sausage is browned and fully cooked through. If there is too much oil in the skillet (from the sausage), drain off some. Leave about 1-2 tbsp for flavor.
Lower the heat to medium-low and add the marinara. Add about 1 cup of water to the marinara jar, shake, and add to the skillet. Give everything a good stir.
Cook until the marinara is heated through. Add the cooked tortellini and stir until the pasta is coated in the marinara. Remove from heat.
Sprinkle the mozzarella over the top of the skillet. Then sprinkle the parmesan over the top.
Bake for about 5-10 minutes until hot and bubbly. If desired, turn the broiler on low and broil for a few minutes to get a nice browning on the top (watch carefully to not burn).
Top with fresh parsley and serve with some crusty bread. Enjoy!!