Melt the butter in a saucepan. Peel and finely chop the onion and add to the pan. Fry gently for about 5 minutes until the onion is soft and tender but not brown. Peel and dice the potato and add to the pan along with the chicken stock. Bring up to the boil, lower the heat and simmer gently for about 15 minutes or until the potato is tender.
Meanwhile wash and trim the sprouts or tops.