Heat oven to 400ºF.
Brown meat with onions in large skillet on medium-high heat; drain. Return meat mixture to skillet; stir in next 4 ingredients.
Arrange 3 tortillas in single layer on bottom of 13x9-inch baking dish. Cover with layers of half of the meat mixture, sour cream and cheese. Repeat layers; cover.
Bake 40 min. or until casserole is heated through and cheese is melted, uncovering after 30 min. Let stand 5 min. before cutting to serve.