Heat a large skillet over medium-high heat. Add the ground beef and diced onion.
Cook for 6–8 minutes, breaking the beef into small pieces, until browned and the onion is softened. Drain excess grease.
Add the minced garlic and cook for about 30 seconds until fragrant.
Transfer the beef mixture to a 6-quart slow cooker.
Add the bell pepper, kidney beans, black beans, diced tomatoes, tomato sauce, tomato paste, and beef broth.
Sprinkle in the chili powder, cumin, smoked paprika, oregano, salt, black pepper, and cayenne pepper.
Stir everything together until well combined.
Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours.
Stir before serving and taste. Adjust the salt or spices if needed.
Serve hot with your favorite toppings.