Preparation Melt the butter in a saucepan and add the potatoes, peeled and cut into quarters, and the onion. Cover and simmer for about 10 minutes. Stir in the stock, salt, pepper and nutmeg, cover and bring to the boil, stirring continuously. Reduce heat and simmer for 30 minutes, stirring occasionally. Remove from heat, filter through a sieve and return to the saucepan. Stir in the milk and cornflour and bring to the boil, stirring continuously. Serve hot with a sprinkling of chives or parsley.