Despite my initial reluctance to try making this soup (see Cafe Volksgarten Pavillon), I made two batches tonight: one with green asparagus, and one with white. There is no appreciable taste difference between the two, and indeed both soups tasted good.
500 grams (18 oz) asparagus
500 ml (ca. 2 1/8C) vegetable stock or asparagus broth
30 grams (1.1 oz) butter, divided
1 pinch sugar
2 tablespoons flour
100 ml (a little less than 1/2C) white wine
1 egg yolk
200 ml (a little less than 1C) cream
3 pinches of nutmeg
3 pinches of pepper (white)
1 teaspoon salt
1. Peel the asparagus and then cut off the heads. Cut asparagus into pieces about 2 cm (3/4″) long.
2. Bring the vegetable/asparagus broth to a boil, then add 1t butter and a pinch of sugar.
3. Add the asparagus pieces to the broth and cook for about 20 minutes. When the asparagus is very tender, remove the soup from the heat.
4. Puree the asparagus and broth mixture.
5. Melt the remaining butter in a sauce pan, then add the flour and fry until the flour turns brown, whisking constantly.
6. Slowly pour in the white wine and whisk. Once the wine is incorporated, add the pureed asparagus soup and bring to a boil. Add the asparagus tips and simmer for about 12 minutes.
7. Whisk the egg yolk with 100 ml cream, then whisk in the remaining cream until the mixture is thick. Gently stir the egg/cream mixture into the soup.
8. Season the soup with lemon juice, pepper, nutmeg, and salt.