Sweat the onions in 2 tablespoons butter. Pour on the Riesling and the poultry stock. Put the lid on halfway and reduce the stock by half.
Form a dumpling out of 1 tablespoon flour and 1 tablespoon butter. Beat the dumpling under the broth with a whisk, let it simmer for 5 minutes. Then stir in the cream and bring to the boil again briefly.
Season to taste with salt and freshly ground white pepper. Finally, alloy the soup with the egg yolks, do not let it boil any more.
For the croutons, cut the toast into small cubes and fry them in hot butter until golden brown. Put the soup in plates and serve with the croutons and fresh chervil.