Heat a 12-inch skillet over medium-high heat. Add the ground beef and diced yellow onion. Cook, breaking up the meat with a spoon, until the beef is no longer pink and the onion is softened.
Drain off and discard any excess fat from the skillet.
Transfer the cooked beef and onion to the slow cooker.
Add the beef stock, frozen corn, the cans of fire-roasted diced tomatoes, crushed tomatoes, drained and rinsed kidney beans, drained and rinsed black beans, diced green chiles, taco seasoning, dry Ranch seasoning, and chipotle chili powder to the slow cooker. Stir to combine.
Cover the slow cooker and cook on low for 4–6 hours or on high for 3–4 hours.
Right before serving, stir in the juice from 1/2 lime and season with kosher salt to taste. If the soup is too thick, add additional beef stock a little at a time to reach your desired consistency.
Serve hot, plain or with your choice of toppings.