In a large skillet, brown the ground beef over medium heat. Once the meat is cooked, place it in a slow cooker, but reserve the fat in the skillet.
Add the diced onion to the skillet and saute for a few minutes to soften it. Transfer the onion, along with any remaining cooking liquid, into the slow cooker.
Add the remaining soup ingredients to the slow cooker (tomatoes, broth, corn, pinto beans, and taco seasoning). Cook the soup on low for 6 hours. If you’re pressed for time, it can be done in the slow cooker in as little as 2 hours, or you can simmer it on the stovetop for an hour. The longer cook time just gives the ingredients more time to mesh.
Serve warm with any of the optional toppings.