Peel and wash the potatoes and cut into small cubes. Peel and chop the onions. Heat the fat in a saucepan. Brown onions and potatoes in it. Add stock, bring to the boil and cook for about 15 minutes.
Clean and wash the leek, cut into rings, cook in a little salted water for about 5 minutes and drain. Cut beef ham into strips. Remove 1/3 of the potato pieces. Mash the remaining potatoes in the stock.
Stir in the crème fraiche. Season to taste with salt, pepper and nutmeg. Add leek, potato pieces and beef ham to the soup. Serve sprinkled with coloured pepper.