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.
Notes
Variation: Add sliced mushrooms with the onions, or swap flank steak for sirloin. For extra heat, increase red pepper flakes. Serve over steamed rice or mashed potatoes to soak up the gravy.Storage: Refrigerate in an airtight container up to 4 days or freeze up to 2 months. Reheat gently, adding a splash of broth if needed.This recipe is an original creation inspired by classic Easy Crockpot Pepper Steak Recipe flavors. All ingredient ratios and instructions are independently developed.