Quick Taco Salad

1 lb. lean ground beef
1 cup chopped onions
1 package taco seasoning mix
1/4 cup water
10 cups torn romaine lettuce (1 large head)
1 1/2 cups chopped tomatoes (2 med.)
1 cup shredded cheddar cheese
2 cups tortilla chips, coarsely crushed
1/4 cup ranch dressing

Brown meat in large skillet sprayed with cooking spray on medium-high heat; drain. Return meat to skillet. Add onions; cook 5 mins., stirring occasionally. Add taco seasoning and water; mix well. Bring to a boil. Reduce heat to medium-low; simmer 3 mins. Divide lettuce among 4 plates. Top evenly with meat mixture, tomatoes, cheese, and tortilla chips. Drizzle with dressing.