Heat the butter in a saucepan over medium–high heat, add the onion and sauté until very tender, about 10 minutes.
Add the carrot to the pan with 100 ml of salted water, then cover and cook for 20 minutes or until tender.
Place in a food processor or blender with caraway and process until smooth, adding more water to reach desired consistency.
Season to taste. Serve soup topped with a dollop of Yoghurt.