Set the Instant Pot to "Sauté," then add the ground beef and diced onion. Cook until the beef is browned and the onion is translucent. If there's excess fat, carefully drain it off.
To the pot, add the chili beans, black beans, corn, tomato sauce, diced tomatoes with green chiles, beef broth, and taco seasoning. Stir to combine all the ingredients well.
Secure the Instant Pot lid and set the valve to "Sealing." Select the manual pressure cooking setting and adjust the time to 10 minutes. Note that it will take about 10 minutes for the Instant Pot to come to pressure.
After the cooking time has finished, perform a quick release by carefully moving the valve to "Venting" to release the pressure.
Stir the soup, then ladle into bowls to serve. Offer optional toppings, like tortilla strips, sour cream, shredded cheese, avocado, and diced green onions for garnish.