Place a large Dutch oven or heavy-bottomed stockpot over medium-high heat.

Add the ground beef and diced onion to the pot. Sauté for 7-9 minutes, breaking the meat into small crumbles with a spatula, until the beef is fully browned and onions are translucent.

Carefully drain and discard any excess rendered fat from the pot.

Sprinkle the taco seasoning and dry ranch dressing mix over the beef mixture. Stir for 1 minute to bloom the spices.

Incorporate the pinto beans, black beans, corn (including liquid), diced tomatoes (including liquid), and Rotel (including liquid) into the pot.

Pour in the beef broth and stir the mixture until the ingredients are thoroughly combined.

Increase heat to bring the soup to a rolling boil, then immediately reduce heat to low.

Simmer the soup uncovered for 15 to 20 minutes, stirring occasionally to prevent sticking and to allow the liquid to reduce slightly.

Adjust seasoning with salt or pepper if necessary based on personal preference.

Ladle into bowls and serve immediately with optional toppings such as shredded cheddar cheese, sour cream, and crushed tortilla chips.