Spread cornstarch over the bottom of a wide, shallow bowl. Dredge flank steak in cornstarch until completely coated; transfer steak to a large resealable plastic bag.

Stir water, soy sauce, brown sugar, carrots, green onions, olive oil, garlic, and ginger together in a bowl; pour into the bag with the beef and seal.

Place the bag of marinating beef into the freezer until the day before you wish to prepare it.

Remove the bag from the freezer. Place it into a bowl in the refrigerator to thaw for at least 24 hours. Empty the bag into a slow cooker. Cover and cook until beef is tender, on Low for 4 to 5 hours or High for 2 to 3 hours.