Clean leek, rinse and cut into long pieces. Steam in the hot vegetable soup for five to ten minutes until tender. Remove with a skimmer, drain a little and place on an oven-proof plate or in a mold.
Mix cheese, egg yolk and sour cream, season with salt, pepper and nutmeg (freshly grated). Pour the cream over the leeks. Crumble pumpernickel and mix with softened butter and chopped parsley. Sprinkle over the leek.
Bake in a heated oven at 200 °C (convection oven 175 °C , gas mark 3) for about fifteen minutes, perhaps covering after eight minutes.
Tips Serve with mashed potatoes or potatoes.
The sauce will be creamier, but also fatter, if you use 125 g mascarpone instead of 75 g sour whipped cream.