In a large skillet, heat the olive oil over medium-high heat. Add the ground beef and cook until browned, breaking it apart as it cooks. Drain any excess fat.
Transfer the browned beef to the slow cooker. Add the diced onion, red bell pepper, green bell pepper, and minced garlic.
Stir in the crushed tomatoes, tomato sauce, tomato paste, and beef broth.
Add the chili powder, ground cumin, smoked paprika, dried oregano, salt, black pepper, and cayenne pepper (if using).
Mix all the ingredients until well combined.
Cover the slow cooker and cook on low for 6-8 hours or on high for 4-6 hours.
Taste and adjust seasonings if necessary before serving.
Serve hot with your favorite toppings such as shredded cheese, sour cream, chopped green onions, or diced avocado.