MICROWAVE meat and onions in 2.3-L microwaveable dish on HIGH 4 to 6 min. or until meat is done, stirring every 2 min. Stir in tomato sauce, water and chili powder. Microwave 5 to 9 min. or until slightly thickened, stirring every 4 min. Transfer to bowl.
PLACE 1 tortilla in same microwaveable dish; top with layers of 1/4 each meat sauce and cheese. Repeat layers 3 times. Microwave 2 to 5 min. or until heated through. Let stand 5 min. Top with tomatoes and sour cream.