If using ground beef or turkey, brown it in a large skillet over medium heat until fully cooked. Drain excess fat and transfer to the slow cooker. If you prefer a healthier option, you can skip this step and add raw meat directly to the slow cooker.
Add the diced onion, minced garlic, and diced bell pepper to the slow cooker.
Stir in the kidney beans, black beans, diced tomatoes with their juice, tomato paste, chili powder, cumin, paprika, salt, black pepper, and cayenne pepper (if using).
Pour in the beef broth or water and stir until all ingredients are well combined.
Cover the slow cooker and set it to low heat. Cook for 6 hours. If you are in a hurry, you can also cook on high for 3 hours.
Once cooked, taste and adjust seasoning if necessary. Serve hot, garnished with your favorite toppings.