Preheat the oven to 375℉.
Place a large skillet over medium heat with enough olive oil to very lightly coat the surface, and sauté the onion until golden brown in spots, about 5 minutes. Add the chili powder and chicken and cook 2 more minutes. Add the tomatoes and corn, cooking 5 more minutes or until heated through.
Spoon about 1 cup of the chicken mixture into a lightly greased 2-quart round casserole. Cover with 3 tortillas, overlapping or cutting to fit the dish, as necessary. Top with 2 cups of the chicken mixture and ½ cup of the cheese. Then layer the remaining 3 tortillas and the last of the chicken mixture.
Tightly cover with foil and bake for 25-30 minutes, or until heated through. Sprinkle with the remaining cheese and return to the oven, uncovered, for 5 more minutes or until cheese is melted. Cut into wedges and serve with the optional garnishes, as desired.