Dice the onion and carrot and sauté with the butter in a saucepan until the onion is translucent. Stir in a tablespoon of flour and deglaze with water. Add the peeled and diced potatoes and fill up with enough water to cover the potatoes.
Add the chopped parsley and cook on a low heat for about 35 minutes. Cut the porcini mushrooms into strips and cook in a pan with a teaspoon of oil and a little water and salt for about 10 minutes.
Puree the soup with a hand blender, season with salt, pepper and 3 tablespoons of cream, then add the porcini mushrooms from the pan and serve.