I also used to use this as a sauce
3 Tunsalted butter
8large leeks (white part only, chopped)
3medium baking potatoes (Stir in 3 medium baking potatoes peeled and thinly sliced)
5 cchicken stock
Bring to a boil, reduce heat and simmer until potatoes are soft ( approx 30 min) pure until smooth. Season with salt. To thin add more chicken stock.