In a large skillet, heat the vegetable oil over medium-high heat. Add the sliced flank steak and sear for about 2-3 minutes until browned. Transfer the steak to the crockpot.
In the same skillet, add the sliced bell peppers, onion, and garlic. Sauté for 2-3 minutes until the vegetables are slightly softened. Add them to the crockpot with the steak.
In a bowl, whisk together the beef broth, soy sauce, black pepper, salt, sugar, and crushed red pepper flakes (if using). Pour this mixture over the steak and vegetables in the crockpot.
Cover and cook on low for 6-8 hours or on high for 3-4 hours until the steak is tender.
About 30 minutes before serving, mix the cornstarch and water in a small bowl to create a slurry. Stir this into the crockpot to thicken the sauce. Cover and continue cooking for the remaining time.
Serve the pepper steak over cooked rice or noodles.