Heat the olive oil in a large saucepan, over medium heat. Sauté the onion, ginger and garlic until softened, about 3 minutes. Stir in the water, carrots and turmeric. Bring the soup to a boil, reduce the heat to low and simmer until the carrots are tender, about 20 minutes. Transfer the soup to a food processor or blender and process with the coconut milk until the soup is smooth. Return the soup to the pan and re-heat. Serve topped with coriander.