Trim roots from leeks. Cut off stem where it shades from light to medium green, and discard darker half. Cut leek in half length-wise, put flat sides down, and slice into 1/4” rings. Rinse rings very well, and pat dry with a towel. Heat 1/2" of vegetable oil in a heavy skillet until it shimmers. Toss leeks with cornstarch to coat. Remove leeks from cornstarch, shake off excess cornstarch and add carefully to oil in a single layer. Cook, stirring once or twice, until pale golden. Remove to absorbent paper and drain. Serve at once.