Slice beef thin against the grain. Season lightly with salt and black pepper.
Heat oil in a skillet over medium-high. Sear beef in batches until browned.
Transfer browned beef to the slow cooker. Add sliced onion on top.
Whisk broth, soy sauce, Worcestershire, brown sugar, garlic, ginger, pepper flakes, and tomatoes.
Pour the mixture over the beef. Cover and cook on Low 5–6 hours.
Stir in bell peppers. Switch to High and cook 45–60 minutes until tender-crisp.
Mix cornstarch with water. Stir into the cooker and simmer 10–15 minutes to thicken.
Taste and adjust seasoning. Serve hot over rice or noodles.