BROWN meat in large nonstick skillet sprayed with cooking spray on medium-high heat; drain. Return meat to skillet. Add onions; cook 5 min., stirring occasionally.
ADD taco seasoning mix and water; mix well. Bring to boil. Reduce heat to medium-low; simmer 3 min.
DIVIDE lettuce among four plates. Top evenly with meat mixture, tomatoes, cheese and tortilla chips. Drizzle with dressing.