Heat oil in pot. Add onions and leeks and sauté.
Add chopped potatoes, chicken stock and water.
Seal pot, boil until potatoes are cooked.
Meanwhile caramelise half leek with sugar and oil.
Blend soup until creamy and smooth. Add sour cream, blend in and drizzle of truffle oil.
Top with caramelised leek, sour cream and rosemary.