In a large stockpot or Dutch oven over medium-high heat, brown the ground beef and chopped onion until the beef is fully cooked and no longer pink.

Drain the excess fat from the pot.

Stir in the taco seasoning and dry ranch dressing mix until the meat is evenly coated.

Add the pinto beans, kidney beans, corn, diced tomatoes, and tomato sauce to the pot. Do not drain the cans; the liquid provides the soup base.

Bring the mixture to a boil, then reduce heat to low.

Simmer the soup for 15 to 20 minutes, stirring occasionally, to allow flavors to meld.

Serve warm with optional toppings such as shredded cheese, sour cream, or tortilla chips.