Put the rice in a large saucepan. Pour over the water and add 1 1/2 teaspoons of the salt. Place the pan over moderately high heat and bring the water to the boil.
Cover the pan, reduce the heat to low and simmer the rice for 15 to 20 minutes or until all the liquid has been absorbed and the rice is tender.
Remove the pan from the heat and set aside.
In a large frying-pan, melt the butter over moderate heat. When the foam subsides, add the onion and fry, stirring occasionally, for 5 to 7 minutes or until it is soft and translucent but not brown.
Add the tomatoes, red pepper, celery and broccoli. Fry, stirring frequently, for 10 minutes or until the vegetables begin to soften.
Add the mushrooms to the pan and fry, stirring frequently; for a further 3 minutes. Season with the remaining salt and the cayenne.
Add the rice to the pan and stir themixture until it is combined. Cook, stirring frequently for a further 10 minutes or until the mixture is thoroughly heated through.
Remove the pan from the heat and serve at once.