In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and cook for 3–4 minutes until softened. Stir in the garlic and cook for 1 minute more.
Sprinkle in the taco seasoning and cumin, then stir to coat the onions and garlic. Pour in the chicken broth and bring the mixture to a simmer.
Add the shredded chicken, black beans, kidney beans, corn, Rotel, and salsa. Stir everything together and bring the soup to a gentle boil.
Reduce the heat and let the soup simmer for 15–20 minutes to allow the flavors to meld. Taste and season with salt and pepper as needed.
Ladle the hot soup into bowls and top with your favorite taco toppings—cheese, sour cream, avocado, cilantro, or crushed tortilla chips.