In a large skillet, cook beef and onion over medium heat until meat is no longer pink; drain. Stir in the water, chili powder, salt, pepper and garlic powder. Bring to a boil. Reduce heat; simmer, uncovered, for 10 minutes. Place 1/4 cup salsa each in two greased 8-inch square baking dishes. Layer each dish with a fourth of the tortillas and 1/4 cup salsa. Divide the meat mixture, sour cream and corn between the two casseroles. Top with remaining tortillas, salsa and cheese. Cover and freeze one casserole for up to 1 month. Cover and bake second casserole at 350° for 35 minutes. Uncover; bake 5-10 minutes longer or until heated through. To use frozen casserole: Thaw in the refrigerator for 24 hours. Remove from the refrigerator 30 minutes before baking. Bake as directed above.