Preheat the oven to 350 degrees. Grease a 7 x 11 or 9 x 13 inch pan with cooking spray.
Start a large pot of salted water to boil. Once boiling, add the tortellini and cook to al dente, about 9 minutes. Drain well, then pour it into the prepared pan.
Meanwhile, brown the beef. Heat the oil in a large non-stick skillet over medium high heat. Add the ground beef, breaking it up into big chunks and spreading evenly across the skillet. Brown undisturbed for a few minutes, then stir and continue to cook until no pink remains. Drain off any excess grease, then season to taste with salt and pepper.
Stir the marinara sauce into the ground beef, heating for about one minute, then add the beef mixture to the tortellini in the pan, stirring until it’s well combined.
Sprinkle the cheese on top then bake for 25-30 minutes or until cheese is melted and casserole is bubbling.