In a large skillet, brown the ground beef and chopped onion over medium-high heat. Drain off any excess grease.
Stir in the taco seasoning, refried beans, diced tomatoes and green chilies, and salsa. Cook for 5 minutes, stirring occasionally, until heated through.
Spread half of the meat mixture evenly in the bottom of a 9×13 inch baking dish.
Layer half of the tortilla wedges over the meat mixture.
Sprinkle half of the cheddar and Monterey Jack cheeses over the tortillas.
Repeat the layers with the remaining meat mixture, tortilla wedges, and cheeses.
Bake for 20-25 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Let stand for 5 minutes before serving. Serve with your favorite optional toppings.
Notes
For a freezer-friendly version, assemble the casserole (without baking) and cover tightly with plastic wrap and then foil. Freeze for up to 3 months. Thaw overnight in the refrigerator and bake as directed, adding a few extra minutes to the baking time if needed.
You can also adapt this recipe for a slow cooker. Brown the beef and onion, then combine all ingredients except the top layer of cheese in the slow cooker. Cook on low for 4-6 hours or on high for 2-3 hours. Sprinkle the cheese on top during the last 30 minutes of cooking.