Melt butter and oil in a large saucepan over very low heat. Add onions and leek, and sauté for approximately seven minutes.

Add sweet potato, potato, and garlic. Cover pan and cook for 10 minutes. Add water and continue simmering until vegetables are tender.

Let cool slightly. Add salt and thyme. Blend, using an immersion blender, until smooth.

Before serving, heat cream slightly and add to the pan. Mix well and serve.