Brown onion in a large skillet. Add chili powder and chicken and cook for two more minutes. Add tomatoes and corn, cooking five more minutes or until heated through.
Spoon about one cup of the chicken mixture into a 2-quart round casserole. Cover with three tortillas, overlapping or cutting to fit dish, as necessary. Top with two cups of the chicken mixture and 1/2 cup of the cheese. Then layer the remaining three tortillas and the last of the chicken mixture.
Cover with foil, and bake at 375 degrees for 25-30 minutes, or until heated through. Sprinkle on the remaining cheese and return to the oven, uncovered, for five more minutes or until cheese is melted. Cut into wedges and serve with the optional garnishes, as desired.