Slice the sirloin or flank steak into strips about 1-inch wide. Season with salt and black pepper.

Cut bell peppers and onion into large chunks (roughly 1-inch pieces). Arrange them in the slow cooker.

In a bowl, whisk together soy sauce, beef broth, minced garlic, brown sugar, and red pepper flakes.

Layer sliced beef in the slow cooker, add the peppers and onion on top, and pour the sauce evenly over them.

Cover and cook on low for 6-7 hours, or on high for 3-4 hours until beef is tender.

30 minutes before serving, mix cornstarch with cold water to create a slurry and stir it into the slow cooker.

Once cooked and thickened, serve the Pepper Steak over rice, noodles, or with bread.