Add sliced flank steak and cornstarch to a large resealable plastic bag. Close and shake to coat well. This can be done in a large mixing bowl as well, but I find it's less messy to do in a bag.
To the bottom of slow cooker, add vegetable oil, garlic, ginger, water, soy sauce, brown sugar and Sriracha sauce. Stir to combine.
Add in coated flank steak and carrots and stir again until everything is coated in the sauce.
Cover and cook on HIGH for 2-3 hours or on LOW for about 4 hours, until steak is cooked and tender.
Serve over white rice and garnished with green onions and sesame seeds if desired.