HEAT oven to 375ºF.
BROWN meat with onions and chili powder in large skillet on medium-high heat. Add tomatoes and corn; cook 5 min. or until heated through, stirring occasionally.
SPOON 1 cup meat mixture into 2-qt. round casserole; cover with 3 tortillas, overlapping if necessary to fit in dish. Top with 2 cups of the remaining meat mixture and 1/2 cup cheese. Cover with remaining tortillas and remaining meat mixture; cover.
BAKE 25 to 30 min. or until heated through. Top with remaining cheese; bake, uncovered, 5 min. or until melted. Cut into wedges. Serve topped with sour cream.