Taco Soup

1 large onion, diced
1 (14 1/2 ounce) can diced tomatoes
1 (14 1/2 ounce) can diced tomatoes & green chiles
1 (11 ounce) canned corn with red and green peppers
1 (15 1/2 ounce) can low-fat or fat-free canned refried black beans
2 cans water (3 1/2 cups)
1 package taco seasoning mix
1 package ranch salad dressing mix (dry)

Saut