For classic German white asparagus soup, you first cook the asparagus in water, and then make a light-coloured roux to which the asparagus cooking liquid is added. To thicken, an egg yolk is added at the end.
Peel the asparagus with a vegetable peeler from top to bottom, starting below the tips. Cut off any woody ends with a knife. Cut the asparagus spears into bite-size pieces. Bring 3 cups of water to the boil in a saucepan, add a pinch of salt and sugar and cook until the asparagus is tender, about 15 minutes. Drain the asparagus, retaining the cooking liquid.
Heat 40g butter in the saucepan. Once melted, add the flour and cook, stirring, until lightly coloured. Whisk in the asparagus cooking liquid a little at a time, and add the stock granules and the cooked asparagus. Bring to the boil.
Whisk the egg yolk and the cream in a small mixing bowl. Turn the temperature down to the lowest setting and let the soup reduce in temperature for a couple of minutes, then whisk in the the egg yolk. If the soup is too hot, the yolk will curdle. Add 75g butter. Season with salt and pepper. Ladle into soup bowls and garnish with chopped parsley.