Heat a large pot over medium-high heat. Add the ground beef and chopped onion and cook, breaking up the meat, until beef is browned and onion is softened, about 5–7 minutes.
Drain any excess fat from the pot.
Add the kidney beans, black beans, corn, diced tomatoes, tomato sauce, and taco seasoning to the pot; do not drain the canned ingredients unless you prefer to.
Stir to combine, bring to a gentle simmer, then reduce heat to medium and cook for about 10 minutes to let the flavors meld.
Ladle soup into bowls and serve with shredded cheddar, sour cream, diced tomatoes, avocado, corn chips, lime wedges, and cilantro as desired.