Cream Soup of Potatoes

Clean the potatoes, carrots and onions and boil them in salted water until tender. Vegetables are cooked enough when you can stick up easily with a fork. It should be cooked thoroughly to get eventually smooth, not crunchy potato soup. Blend cooked vegetables in a blender or food processor until a uniform mixture. Do not throw away the water in which vegetables are boiled. Add the mixture to the butter, milk and half a liter of water in which vegetables are boiled (do not put a lot of water to not make the soup skilly). Mix well to obtain a homogeneous mixture and return for another 5 minutes on the stove.