Heat oven to 400 degrees F.
Brown meat with onions in large skillet on medium-high heat; drain. Stir in salsa, beans, dressing and seasoning mix.
Arrange 3 tortillas on bottom of 13-by-9-inch baking dish; cover with layers of half each meat mixture, sour cream and cheese. Repeat layers. Cover with foil.
Bake 40 minutes or until casserole is heated through and cheese is melted, removing foil after 30 minutes. Let stand 5 minutes before cutting to serve.