How to Make It
Heat oil in a large Dutch oven over medium-high. Add half of steak; cook, stirring occasionally, until browned, about 6 minutes. Remove steak from pan. Repeat procedure with remaining steak. Set steak aside. Add onion and garlic to pan; cook, stirring occasionally, until onion is tender, about 6 minutes.
Stir in chili powder, chipotle chile, and cumin; cook, stirring constantly, 1 minute. Add kidney beans, tomatoes, stock, pinto beans, and salt; bring to a boil. Return steak to pan; cover, reduce heat to low, and simmer 30 minutes. Stir in bell peppers; cook until steak is tender, about 30 minutes. Ladle chili into bowls; top with avocado, yogurt, scallions, cilantro, and, if desired, radishes.
Slow Cooker Method - Prepare as directed in step 1, substituting a large skillet for the Dutch oven. Transfer steak and onion mixture to a 6-quart slow cooker. Add chili powder, chipotle, cumin, kidney beans, tomatoes, stock, pinto beans, salt, and bell peppers. Cover and cook on LOW until beef is tender, 5 to 6 hours. Serve with avocado, yogurt, scallions, cilantro, and, if desired, radishes.