Heat the olive oil in a large saucepan. Saute garlic for 3 minutes, then add the potato, cornflour and dried rosemary. Saute 2 more minutes, without burning.
Add water and half of the corn. Bring to a boil, reduce to a simmer, cover and cook 25 minutes over low heat. Blend the soup.
Add the rest of the corn and simmer 15 minutes more until corn is just tender. Pour into bowls and garnish with a sprig of rosemary.