Clean the leek, wash it and possibly cut it in half. Blanch in boiling salted water for about 4 minutes. Drain well, collecting 300 ml cooking water. Cut the ham in half. Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see
Finely grate the cheese. Heat butter in a pot. Lightly sauté the flour in it. Pour in cooking water and cream while stirring and bring to the boil. Simmer for about 5 minutes. 1⁄3 Melt cheese in the sauce. Season with salt, pepper and nutmeg.
Grease a fat pan or large casserole dish with oil and spread the leeks on top. Pour cheese sauce over it. Sprinkle with the rest of the cheese. Spread the ham on top. Bake in a hot oven for 20-25 minutes.
Mashed potatoes or boiled potatoes taste good with it.