Place the mushrooms in a bowl and pour boiling water over them. Leave to soak for 30 minutes.
Melt the butter in a saucepan and add the onion, then cook over a medium heat for about 5-7 minutes until it starts to soften.
Put the rice into the pan and cook the onion and rice together for about 1 minute stirring constantly.
Pour in the soup and the soaked mushrroms along with any remaining liquid in the bowl.
Reduce the heat and simmer for about 15 minutes stirring occasionally until thickened and the rice is tender. (If its too dry add a little boiling water, if its too runny boil rapidly to evaporate excess liquid.)
Season to taste with salt and pepper; spoon onto warm plates and sprinkle with the Parmesan cheese.