Wash the leeks well and pat dry. In a large saucepan, heat 1/2 inch of oil until shimmering. Add all but 1/2 cup of the leeks and cook over moderate heat, stirring, until golden brown, 10 to 12 minutes. Using a slotted spoon, transfer the leeks to a paper towel–lined plate. Discard the oil. Season with salt and pepper.