In a Dutch oven, cook the ground beef or turkey with chopped onion over medium heat, stirring until the meat is browned. Drain off any excess grease.
Stir in the canned tomatoes, kidney beans, hominy, taco seasoning, and ranch dressing mix. Make sure everything is well combined.
Reduce the heat to low and let the soup simmer gently for about 2 hours, stirring occasionally. Add 1–2 cups of water if you prefer a thinner consistency.
Ladle the soup into bowls and top with shredded Cheddar cheese and a spoonful of sour cream, if you like.