The Elixir Soup is a potion given to Link by his Grandma on Outset Island in The Wind Waker. It fully restores Link's health and magic in addition to doubling his attack strength until Link receives any damage. The soup is first available when Link stops on Outset Island in search of Jabun.
If Link heals his sick Grandma with a Fairy, she'll give Link the Elixir Soup. Link can visit his Grandma and get unlimited refills of the soup (as long as Link has a free bottle), but Link is only allowed to have one bottle of it at a time. Each bottle of soup has two servings.