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.
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.