Potatoes: Cook in lightly salted water 15-18 min until tender. Drain and let evaporate for 1 min on low heat.

Wash leeks: Halve lengthwise, rinse thoroughly between layers (sand!), chop finely.

Stew leeks: stew 8-10 min in 10 g butter (and/or dash of water) with lid until soft, do not color. Last minutes without lid to allow excess liquid to evaporate. You can choose to blend the leeks.

Purée: Mash potatoes with warm milk, possibly an egg and butter. Season with salt, pepper, nutmeg and cayenne pepper.

Mix: Fold in the stewed leeks. Taste and finish with mustard if necessary.