Place shucked corn on a microwave-safe plate, and cover with wax paper. Microwave at HIGH 5 minutes or just until tender. Cut kernels from ears of corn to measure 1 cup.
To prepare salsa, combine the tomato and next 6 ingredients (tomato through tomato sauce) in a bowl; set aside.
To prepare the chicken, combine water, oregano, cumin, 1/4 teaspoon salt, and sliced garlic in a large saucepan; bring to a simmer. Add chicken; partially cover, reduce heat, and simmer 20 minutes or until done. Remove chicken from cooking liquid, reserving cooking liquid, and cool slightly. Shred chicken with 2 forks; place in a bowl. Bring cooking liquid to a boil; cook until reduced to 1 1/2 cups (about 9 minutes). Cool; discard sliced garlic.
Preheat oven to 375°.
To prepare the sauce, place flour, chili powder, 1/4 teaspoon salt, and red pepper in saucepan, and gradually add the cooking liquid, stirring with a whisk until blended. Cook over medium heat 4 minutes or until thick. Stir 3/4 cup sauce into chicken in bowl.
Spread 3/4 cup salsa in bottom of a 13 x 9-inch baking dish coated with cooking spray. Arrange 8 tortilla halves over salsa; top with 3/4 cup salsa, half of chicken mixture, half of corn, and 1/4 cup cheese. Repeat layers, ending with 8 tortilla halves. Spread the remaining salsa over the tortilla halves, and top with remaining sauce. Bake at 375° for 35 minutes. Sprinkle with 1/4 cup cheese, and bake an additional 5 minutes or until thoroughly heated. Let the casserole stand for 10 minutes before serving. Top each serving with 1 tablespoon sour cream. Serve with leaf lettuce, if desired.