Wash the potatoes and cook in boiling water for about 15 minutes. Quench cold and peel. Peel and finely dice onion. Heat the fat in a pot and fry the onion cubes in it.
Dust with flour and sweat it. While stirring, deglaze with stock and milk and bring to the boil. Season with salt, nutmeg and marjoram. Cut ham into strips. Add peas and ham to the sauce and simmer over low heat for about 5 minutes.
Add the potatoes. Grate cheese finely. Sprinkle the béchamel potatoes with cheese and bake in an ovenproof dish for about 10 minutes. Serve garnished with marjoram as desired.