Combine flour, cornstarch, salt and pepper in a shallow dish. Trim ends from leek and rinse well, making sure to rinse under any loose leaves. Cut leek into 1/4" slices and separate rings. Toss leek slices in flour mixture to gently coat. Add oil to a shallow pot or skillet and heat until the oil sizzles when a sprinkle of flour is added. Scoop out half of the leek slices shaking off any excess flour and add to oil. Fry for 4-5 minutes until golden, turning once. Using a slotted scoop remove leeks from oil and allow to drain on a paper towel lined plate. Repeat the frying process with remaining leeks. Keep uncovered at room temperature until ready to use.