Put about 3cm of water in a greased frying pan or use a non-stick pan, adding a teaspoon of white vinegar and salt to the water, to prevent the egg white spreading.
Heat the water to simmering point, break egg into a small saucer and slide the egg gently into the simmering water. Cook for 3-5munutes depending on the firmness desired. Lift carefully with a slotted spoon. Serve well drained, on buttered toast.