Mushroom soup

Instructions
Clean the mushrooms, wash, dry and cut them into thin slices. In a pan, melt 15 g butter and put a small onion ground together with a pinch of parsley. When the mixture is wilted, add a tablespoon of tomato sauce, the peeled and sliced potatoes and two sage leaves, salt and let flavour for a few minutes. Cover with 1 liter hot broth, bring to the boil, add the mushrooms and cook for 20 minutes. Arrange in the bowls bread croutons browned in butter, on which you will pour the hot soup directly. Serve with grated Grana cheese separately.