Prepare the leeks; trim off the roots and remove the tough, outer layers. Cut off most of the green part, leaving about 2 in (5 cm) in place. Wash very thoroughly. Cut lengthwise in quarters, then slice each length into 1 - 1 1/4 in (3 cm) sections.

Cook the chopped onion in the oil over a low heat in a very large frying pan or wide saucepan for about 10 minutes, stirring frequently until it is tender and wilted but not at all browned.

Stir in the flour, 1/2 - 1 tsp salt and 1/2 tsp caster sugar and continue stirring for 2 minutes; 1 - 1 1/2 tbsp tomato puree can be added at this point for extra flavour if wished.

Gradually add 14 fl oz (400 ml) hot water, stirring continuously with a wooden spoon or beating with a balloon whisk. Increase the heat to moderately high and keep stirring or beating as the mixture comes to the boil; add the leeks. Mix thoroughly, turn down the heat to very low, cover tightly and simmer gently for 10 minutes.

When this time is up, sprinkle in the rice, stir well, cover tightly again and cook for a further 15 minutes, until the rice is tender. Garnish with lemon wedges.