1 stalk of rhubarb. Aim for ones that are about 25 cm long or more?
Water
a bit of salt and ginger powder, not much
Chop the stalk up, boil and then simmer it in the water for about 30 minutes. Press rhubarb through a strainer, and discard the fiber-y bits. Apparently you are supposed to add sugar or honey to such a soup, but that would make it too sweet for me.