Melt the butter in a saucepan and add the onion. Fry gently for about 5 minutes or until the onion is soft and tender, but not brown. Peel and dice the potato and add to the pan. Toss in the onion-flavoured butter and then stir in the stock. Bring up to the boil, lower the heat and simmer for about 15 minutes or until the potato is quite tender.
Meanwhile separate the lettuce leaves, wa