Finely chop the onions. Halve the leek, wash the leaves under running water, cut into fine strips. Fry the onions in butter until translucent. Briefly sauté the leek, dust with flour. Pour in the stock, season to taste and cook gently for about 15 minutes. Cut the ham into fine strips, stir the processed cheese with the sour cream and add both to the soup. Bring to the boil again and season well with salt, pepper and nutmeg.