Chop the onion, bell pepper, and jalapeño (if using). Mince the garlic cloves so they’re ready to go.
Heat a skillet over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks. Drain off any excess fat.
In the same skillet, add a splash of oil if needed and sauté the onion, bell pepper, jalapeño, and garlic until soft and fragrant, about 5 minutes.
Transfer the browned beef, sautéed vegetables, kidney beans, black beans, diced tomatoes (with juices), beef broth, chili powder, cumin, oregano, salt, and pepper into your slow cooker. Stir well to combine.
Cover and cook on low for 6-8 hours or high for 3-4 hours. The longer it cooks, the more the flavors deepen.
Before serving, taste the chili and adjust salt, pepper, and chili powder to your liking.
Ladle the chili into bowls and garnish with your favorite toppings like shredded cheese, sour cream, chopped green onions, or fresh cilantro.