Cut off the stalk from the asparagus at the lower end approx. 2 cm (if necessary peel the lower third). Then cut into small (1-1.5 cm) pieces. Peel and roughly cut the onion. Roughly cut the mushrooms. Cook everything in a saucepan with approx. 0.8-1l water for approx. 30 minutes. Add the instant stock during the cooking process. Then take the saucepan off the heat, add the sour cream and puree everything well with a hand blender. Season to taste with salt and white pepper (Attention! The heat will increase) and a splash of fresh lemon juice.