Add sliced beef to the crockpot.
Add sliced peppers and onions on top.
Whisk beef broth, soy sauce, brown sugar, garlic, and seasonings; pour over beef and peppers.
Cover and cook on LOW 6–7 hours or HIGH 3–4 hours, until beef is tender.
(Optional) Stir in a cornstarch slurry during the last 20–30 minutes to thicken.
Serve hot over rice.
