Take a pan and let cook the finely cut onion in the butter, add mushrooms cut into small pieces and mix everything well.
Cover everything with the lid and cook over low heat for 15 minutes, stirring occasionally.
Then add the rice and a little bit of white wine and letting it evaporate slowly.
Add the vegetable stock whenever it is necessary until the rice will be cooked.
Pour in the mixture a sachet of saffron and the parmesan cheese just few minutes before the end of cooking and mix all well.