Wash the potatoes thoroughly, possibly halve them. Cook in boiling salted water for about 15 minutes. Cut the smoked pork loin into cubes. Peel onions, cut into slices and fry in hot fat.
Add flour and sauté while stirring. Gradually stir in the stock and milk, bring to the boil and simmer over a mild heat for about 5 minutes. Stir in mustard, season with salt and pepper.
Wash the marjoram, dab dry and, except for a few leaves, stir into the sauce. Drain the potatoes and layer them in a flat casserole dish with the smoked pork loin. Add the sauce and bake in a preheated oven (electric range: 200 °C/ gas: level 3) for approx. 30 minutes.
Sprinkle with the remaining marjoram leaves.