Lightly spray an 11x7x1-1/2 inch baking dish with vegetable oil cooking spray. Preheat oven to 350°F. Sauté the onion in 1/4 cup water in a medium skillet until tender, stirring often. Add the tomato, chicken, broth, chili powder, salt, cumin, and oregano.
Bring to a boil, reduce the heat, and simmer 5 to 6 minutes. Layer half the chicken mixture, half the tortillas, and half the cheese in the baking dish and repeat with the remaining chicken and tortillas.
Reserve the remaining cheese. cover the casserole and bake for 25 to 30 minutes. Uncover and sprinkle evenly with the remaining 1/2 cup cheese. Bake an additional 5 minutes, or until the cheese is bubbly.
Serves 6.
