Step 1: In a skillet over medium heat, brown the ground beef with the diced onion and minced garlic until the beef is fully cooked and the onion is softened, about 8-10 minutes. Drain any excess fat.
Step 2: Transfer the cooked beef mixture to a 6-quart slow cooker.
Step 3: Add the taco seasoning, black beans, kidney beans, corn, diced tomatoes with green chilies, tomato sauce, beef broth, chili powder, and cumin to the slow cooker. Stir well to combine.
Step 4: Season with salt and pepper to taste.
Step 5: Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the flavors are well blended and the soup is hot.
Step 6: Serve hot, topped with optional toppings like shredded cheese, sour cream, tortilla chips, or fresh cilantro.