Preheat your oven to 375 degrees Fahrenheit. Lightly grease a 9×13 inch baking dish.
Cook the ground beef and chopped onion in a large skillet over medium heat until the beef is browned and the onion is soft. Drain off any excess grease.
Stir the tomato sauce, enchilada sauce, water, chili powder, cumin, and garlic powder into the skillet with the beef mixture. Bring the sauce to a simmer.
Add the uncooked cheese tortellini to the sauce mixture in the skillet. Stir gently to coat the tortellini completely.
Pour the entire mixture into the prepared baking dish.
Sprinkle 1 1/2 cups of the shredded cheese evenly over the top of the mixture.
Bake for 25 to 30 minutes, or until the tortellini is tender and the cheese is melted and lightly browned.
Remove from the oven and let it stand for 5 minutes before serving. Top with remaining cheese, sour cream, and cilantro, if desired.
Notes
You can use frozen, fully cooked tortellini; if you do, reduce the baking time by about 10 minutes.
Use mild, medium, or hot enchilada sauce based on your preference for spice level.
For a richer flavor, substitute chicken broth for the water in the sauce mixture.