**Cook the ground beef**
In a large skillet over medium heat, cook the ground beef until browned, breaking it up with a wooden spoon. Add the diced onion and garlic, and sauté until softened, about 3–4 minutes. Drain any excess fat.
**Combine ingredients in the slow cooker**
Transfer the cooked beef mixture to the slow cooker. Add the diced tomatoes, tomato sauce, kidney beans, black beans, beef broth, chili powder, cumin, smoked paprika, oregano, salt, black pepper, and cayenne pepper (if using). Stir everything together until well combined.
**Cook the chili**
Cover the slow cooker with the lid and cook on **low** for 6–8 hours or **high** for 3–4 hours, stirring occasionally if possible.
**Serve**
Once the chili is done, taste and adjust seasoning if needed. Serve hot with your favorite toppings such as shredded cheese, sour cream, chopped green onions, or jalapeños.