Optional: In a large skillet, heat the vegetable oil over medium-high heat. Add the sliced flank steak and brown it for about 3-4 minutes. This step adds extra flavor but can be skipped if you're short on time.
Transfer the browned (or raw) flank steak into the crock pot.
Add the sliced green and red bell peppers, sliced onion, and minced garlic to the crock pot on top of the steak.
In a separate bowl, mix the beef broth, soy sauce, Worcestershire sauce, cornstarch, black pepper, salt, and red pepper flakes (if using). Stir until the cornstarch is dissolved.
Pour the sauce over the steak and vegetables in the crock pot, ensuring everything is well coated.
Cover the crock pot and cook on low for 6 hours or on high for 3 hours, until the steak is tender.
Once cooked, give it a gentle stir and serve over cooked rice.