Microwave meat and onions in 2-qt. microwaveable dish on HIGH 4 to 6 min. or until meat is no longer pink, stirring every 2 min.; drain.
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.
