Clean the leek, cut lengthwise, wash thoroughly and cut into pieces of about 5 cm. Cover and stew in little boiling salted water for about 5 minutes. Drain well.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Cut the ham in half, wrap each one around the leek and place in a greased casserole dish. Grate cheese and mix with the sour cream.
Spread on the leek. Bake in a hot oven for about 20 minutes. Mashed potatoes taste good with it.