Brown the beef (optional but recommended): In a large skillet over medium heat, cook the ground beef until browned, breaking it into crumbles as it cooks. Drain any excess fat. Transfer the cooked beef to your slow cooker.
Sauté the onion and garlic (optional): In the same skillet, add the diced onion and garlic. Sauté for 2-3 minutes until softened, then transfer to the slow cooker with the beef. (You can skip this step and add the raw onion and garlic directly to the slow cooker, but sautéing enhances their flavor.)
Combine the ingredients: Add the kidney beans, black beans, diced tomatoes, tomato sauce, tomato paste, and beef broth to the slow cooker. Stir in the chili powder, cumin, smoked paprika, oregano, cayenne pepper (if using), salt, and black pepper.
Cook: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours. The longer the chili cooks, the more the flavors will develop.
Adjust seasoning: Taste the chili and adjust the seasoning if needed, adding more salt, pepper, or spices to your preference.
Serve: Ladle the chili into bowls and top with your favorite toppings like shredded cheese, sour cream, diced avocado, green onions, or tortilla chips.
