Brown the beef: In a large skillet over medium-high heat, brown the beef on all sides. Remove from the skillet and set aside.

Sauté vegetables: In the same skillet, sauté the onion, garlic, and green bell pepper until softened, about 5 minutes.

Combine ingredients: Transfer the beef, kidney beans, black beans, diced tomatoes, tomato sauce, chili powder, cumin, oregano, cayenne pepper, salt, and pepper to a slow cooker. Stir to combine.

Cook: Cook on low for 6-8 hours, or until the beef is tender and the flavors have melded.

Serve: Serve hot with your favorite toppings, such as shredded cheese, sour cream, chopped onions, or avocado.

Optional additions: For a spicier chili, add a pinch of red pepper flakes or a teaspoon of hot sauce. For a thicker chili, add a tablespoon of cornstarch mixed with 2 tablespoons of water to the slow cooker.