Cover the chestnuts with boiling water and leave them to soak for several hours, then cook them until tender.
(If the chestnuts are soaked in warm water in a wide-rimmed thermos flask, or in a warm cupboard, them may be tender enough not to require more cooking.)
Chop the onion and saute it in the oil in a saucepan for about 3 minutes.
Skin and chop the tomato.
Slice the mushrooms. Add them to the pan and cook for a further 3 minutes or so.
Add the rice and chestnuts to the pan and stir well.
Then add the water, yeast extract and tomato paste.
Mix together very thoroughly as it heats up so that the yeast extract is amalgamated evenly into the mixture.
Continue cooking over a gentle heat until all the ingredients are well heated.