In a large skillet, heat the vegetable oil over medium-high heat. Add the sliced flank steak and sear until browned on all sides, about 3-4 minutes. Remove from heat and transfer to the crockpot.
In the same skillet, add the sliced onion and bell peppers. Sauté for 2-3 minutes until slightly softened. Add the minced garlic and cook for an additional minute.
Pour the sautéed vegetables over the steak in the crockpot. In a separate bowl, whisk together the beef broth, soy sauce, cornstarch, water, black pepper, sugar, and salt. 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.
Before serving, stir the mixture to combine. Serve over cooked rice.