Preheat the oven to 350°F and spray a 9x13 baking dish with cooking spray.
Brown the ground beef and diced onion in a skillet over medium-high heat until the beef is fully cooked and the onion is soft. Drain well.
Add the taco seasoning and refried beans to the beef mixture. Stir until fully combined and warmed through.
In a separate bowl, stir together the sour cream and cream of mushroom soup until smooth.
Cut two tortillas in half so you can fit them around the edges of the baking dish.
Spread half the sour cream mixture in the bottom of the pan. Layer with tortillas, half the beef mixture, and 1 cup cheese.
Repeat with the remaining sour cream mixture, tortillas, beef mixture, and cheese.
Cover tightly with foil and bake for 20 minutes. Remove foil and broil for 3 to 5 minutes, watching closely, until the cheese is golden and bubbly.
Let the casserole sit for 5 minutes before cutting.
Notes
Drain the beef well so the casserole does not get greasy. Six-inch tortillas fit the pan better than burrito-size tortillas. Full-fat sour cream holds up better during baking. Let the casserole rest for 5 minutes before slicing so the layers hold together.