Heat a skillet over medium heat. Add ground beef and chopped onion, cooking until the beef is browned and the onion is tender, about 7-8 minutes. Drain excess fat, then stir in taco seasoning and refried beans. Heat through for 3-4 minutes.

In a bowl, mix cream of mushroom soup and sour cream until smooth.

Spread half of the soup mixture on the bottom of a baking dish. Tear tortillas into pieces and layer them on top, covering the soup.

Spoon half of the beef mixture over the tortillas, then sprinkle 1 cup of shredded cheese. Repeat with remaining soup, tortillas, beef, and cheese.

Bake at 350°F for 20 minutes, or until the cheese is melted and bubbly, and the edges are golden brown.