Clean, wash and cut the leek into pieces of about 10 cm. Cook in boiling salted water for about 5 minutes. Remove and drain well. Melt the fat in a pot. Stir in flour and sweat briefly.
Add the stock and milk while stirring. Bring to the boil, boil for 3-5 minutes. Stir in processed cheese and season to taste with salt and pepper. Grate cheese. Wrap the leek pieces with roast and put them into a large greased casserole dish or 4 small moulds.
Pour sauce over it and sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes. Wash parsley, shake dry, pluck leaves from the stalks and chop.
Sprinkle parsley over the finished casserole.