Melt the butter in a pan and add the onion and the potatoes. Cook until transparent and then add the stock and seasoning, bring to the boil and cook gently until the vegetables are tender. Liquidise and put into a clean pan.
Take off the red coral part of the cleaned scallops and slice. Heat the milk and poach the white part of the scallops for 4 minutes. Remove from the milk and slice very thinly.
Beat the egg, add the hot milk and the cream. Mix with the potato purée and heat, but take care not to boil. Add the scallops, including the red coral, allow to heat again and serve with chopped parsley.