1.Place Flank Steak And Cornstarch Into A Resealable Plastic Bag. Shake The Bag To Evenly Coat The Flank Steak With The Cornstarch. Allow To Steak Rest For 10 Minutes. 2.Heat Olive Oil In A Large Skillet Over Medium-High Heat. Cook And Stir Steak Until Evenly Browned, 2 To 4 Minutes. Place Onion, Garlic, Flank Steak, Green Onions, Soy Sauce, Water, Brown Sugar, Ginger, And Hoisin Sauce In A Slow Cooker. Cook On Low Setting For About 4 Hours.