Wash the potatoes and cook in boiling water for about 15 minutes. In the meantime, clean, wash and cut leek into rings. Cut smoked pork loin into pieces. Drain potatoes, rinse with cold water, peel and cut into slices.
Fill the potatoes, leek and celery in a greased casserole dish. Peel and finely dice the onion. For the béchamel sauce melt the fat in a pot. Briefly sauté the onion in it, dust with flour and sauté.
Deglaze with milk and cream while stirring and bring to the boil. Stir in mustard and season to taste with salt and pepper. Pour the sauce over the casserole, sprinkle with cheese and bake in a preheated oven (electric cooker: 200 °C/ gas: level 3) for approx. 20 minutes.
Garnish with parsley as desired.