In a saucepan, melt the butter and add the leeks. Stir-fry for a few minutes until they are soft.
Add the mushrooms and the potatoes. Cover with a lid and leave it for about 3-4 minutes. Add salt and black pepper to taste. Pour over hot, pre-prepared vegetable broth (or hot water). Let it boil for about 20 minutes until the potatoes are cooked.
Blend everything and cook it for another 2-3 minutes. Season with salt if it is necessary.
Serve the soup hot, garnished to taste with croutons, olive oil, stir-fried mushrooms or leeks.