In a skillet over medium heat, cook the ground beef with the diced onion until the beef is browned and the onion is softened, about 5 to 7 minutes. Drain any excess fat.

Add the garlic and cook for 30 seconds, just until fragrant.

Transfer the beef mixture to the crockpot.

Add the bell pepper, kidney beans, black beans, diced tomatoes, tomato sauce, tomato paste, chili powder, cumin, smoked paprika, salt, black pepper, cayenne if using, and beef broth. Stir well to combine.

Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the flavors are blended and the chili is thickened.

Taste and adjust seasoning if needed before serving.

Serve hot with your favorite toppings, if desired.