Heat
the oil in a large saucepan. When it is hot, add the
onion and fry for 1 minute, stirring constantly. Add
the cucumber and fry for 1 minute.
Stir
in the stock and bring it to the boil. Reduce the
heat to low and simmer for 10 minutes, stirring
occasionally. Stir in the tomato quarters and simmer
very gently for a further 5 minutes.
Remove the pan from the heat and using a whisk or
fork, carefully whisk the egg into the soup. Serve
at once.