Blanch onions in a 2- to 3-quart pot of boiling salted water (see
Tips) 1 minute, then transfer with a slotted spoon to a bowl of cold water to stop cooking. Drain onions and peel, trimming root ends with a paring knife. Wash leek slices in a bowl of water, agitating them, then lift out and drain in a medium-mesh sieve.