Put the dried mushroom in a bowl and pour over the water. Leave the mushroom to soak for 30 minutes. Strain off but save water. Cut the mushroom into small pieces.
Mix the soaking water with the foundation in a jug. Heat the oil in a saucepan and fry the onion softly without giving it any color. Add rice, mushrooms and saffron.
Pour in half of the mushroom broth and simmer without lid 25-30 minutes. Stir and dilute continuously over time with the broth.
Add butter, cheese and parmesan. Stir and season with salt. Serve with extra parmesan.