In a large skillet, pour some olive oil down & brown beef on both sides (5-10min). Place onion and green pepper in a bottom of slow cooker. Top with beef. Combine broth, Worcestershire sauce, thyme, pepper, garlic if desired and pepper flakes; pour over beef. Cover and cook on high until meat reaches desired doneness and vegetables are crisp-tender, about 3.5-4 hours.

Optional: Remove beef and keep warm. Combine cornstarch and water until smooth; gradually stir into cooking juices. Cover and cook on high until slightly thickened, about 30 minutes. Return beef to the slow cooker; heat through.