Preheat the oven to 400℉. Spray the inside of a 9x13-inch casserole dish with olive oil cooking spray.
Cook the tortellini according to the package directions. Drain well. Add the drained tortellini and marinara back to the pot that you cooked the tortellini in. Toss to coat the tortellini with the sauce. Stir in 1 cup of the shredded mozzarella.
Transfer the tortellini mixture to the prepared casserole dish, and spread it out in an even layer. Top the tortellini with the remaining 3 cups shredded mozzarella and the grated parmesan.
Cover the dish with foil. Bake until the cheese is melted, about 20 to 25 minutes. To brown the top a bit, remove the foil and broil a couple minutes.
Sprinkle the parsley on top, and serve.
Notes
Cheese-filled tortellini (cheese or spinach-cheese) works best.
Add cooked sausage, ground beef, or rotisserie chicken to make it heartier.
Stir in a handful of spinach or mushrooms for extra veggies.
Store leftovers in the refrigerator for up to 4 days and reheat before serving.