Brown meat with onion& chile powder in a large skillet on medium high heat. Add tomatoes and corn. Cook 5 minutes more or until heated through.
Spoon 1 cup meat mixture into a 2 quart round casserole dish. Cover with 3 tortillas. Overlapping if necessary to fit dish. Top with with a little more meat mixture.
Top with 1/2 cup cheese. Cover with 3 more tortillas and remaing meat mixture. Cover.
Bake at 375º for 25 -30 minutes. Top with remaing cheese. Bake uncovered 5 more minutes. Serve with sour cream.