Place the sliced flank steak in a large bowl. Add cornstarch and toss to coat evenly.
Heat vegetable oil in a large skillet over medium-high heat. Sear the beef in batches until browned, about 1-2 minutes per side. Transfer to the slow cooker.
In a bowl, whisk together soy sauce, water, brown sugar, garlic, ginger, and red pepper flakes. Pour the mixture over the beef in the slow cooker.
Cover and cook on low for 4-5 hours or high for 2-3 hours, until the beef is tender.
Serve over cooked rice and garnish with green onions.