Peel, wash and halve the potatoes. Cook in boiling salted water for 15-20 minutes. In the meantime clean and wash the leek and cut it into bite-sized pieces. Cook in 100 ml boiling salted water for about 5 minutes.
Then drain and store the stock. Melt the fat for the sauce. Sweat flour in it. Add stock and cream while stirring constantly, bring to the boil. Melt cheese triangle in it. Season to taste with salt, pepper and nutmeg.
Wash the chives and cut into small rolls. Grate the gouda. Fill prepared vegetables and ham into a small form. Spread the sauce on top. Sprinkle with gouda. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for about 12 minutes.
Serve sprinkled with chives.