In a skillet, cook the ground beef. Spray the 9x13 pan with cooking spray.
In a large bowl mix taco seasoning, refried beans, and cooked ground beef. Place half of the meat mixture into the 9x13 pan and smooth so that it's even.
Place a layer of tortillas. Place a layer of cheese. (About 1.5 cups.) Repeat layers. Top with diced tomatoes and onions.
Bake 375 degrees Fahrenheit for 45 minutes to 1 hour.