Brown the meat (optional but recommended): In a skillet over medium heat, cook the ground beef or turkey with the diced onion until the meat is no longer pink. Drain excess fat. Stir in the garlic and cook 1 minute.
This step adds flavor and keeps the soup from getting greasy.
Add to the crock pot: Transfer the cooked meat and onions to the slow cooker. Add taco seasoning, cumin, chili powder, beans, corn, diced tomatoes with green chiles, fire-roasted tomatoes, tomato sauce, and 3 cups of broth.
Stir and set: Mix well. Cover and cook on Low for 6–8 hours or High for 3–4 hours.
If it looks too thick, add the remaining 1 cup of broth during the last hour.
Finish and season: Stir in lime juice and cilantro if using. Taste and add salt and pepper as needed. If you want more heat, add a pinch of cayenne or hot sauce.
Serve: Ladle into bowls and top with cheese, sour cream, tortilla chips, and any other favorite toppings.
Serve with warm cornbread or a simple green salad on the side.