Add the spaghetti to a pan of boiling salted water. Push the spaghetti slowly into the water and as it softens wind it round the inside of the pan to keep the strands whole. Bring back to the boil and cook for 12 minutes, then drain.
Melt the butter in the hot saucepan. Add the chopped onion. Cover with a lid and fry gently for a few minutes until the onion is tender but not brown. Add the tomato purée and the water and stir to make a tomato, onion and butter glaze. Draw the pan off the heat. Add the hot cooked spaghetti and using two forks toss and turn the spaghetti so that it is well glazed and coloured. Turn into a hot dish and serve.