In a large skillet, heat the vegetable oil over medium-high heat. Add the sliced flank steak and sear until browned, about 3-4 minutes. Transfer the steak to the crockpot.
In the same skillet, add the sliced onion and bell peppers. Sauté for about 3-4 minutes until they start to soften. Add the minced garlic and cook for an additional minute. Transfer the vegetables to the crockpot.
In a bowl, whisk together the beef broth, soy sauce, cornstarch, brown sugar, black pepper, salt, and crushed red pepper flakes (if using). Pour the 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.
Serve the pepper steak over cooked rice.