Lightly coat a 13 x 9 x 2 baking pan with nonstick cooking spray; set aside.
In large skillet cook meat until brown. Drain fat. Stir in water, taco seasoning and garlic powder. Bring to a boil; reduce heat. Simmer, uncovered, about 5 minutes or to desired consistency; set aside.
In mixing bowl combine cottage cheese, cheddar cheese and onion.
Place 6 tortillas in prepared pan. Top with half of the meat and dollop with half of the cheese mixture. Repeat layers. Bake, uncovered for 40 to 45 minutes or until heated through.
Top with lettuce and tomatoes. Serve with sour cream, if desired.