In a deep pan, place a bit of butter or olive oil and onion cut up in small pieces. Cook over medium flame until the onion becomes transparent (not brown) and then add mushrooms cut in pieces. Cook for some minutes so that the mushrooms are flavored. Salt and (if you like) pepper.
Then add the rice. A very rough measure is three fistfuls of rice per portion.
Cook the rice, stirring with the onions and mushrooms for about 5 minutes.
Then add just enough hot broth the cover the rice which should continue to cook slowly.
The broth is easily made with cubes (I suggest chicken or vegetable instead of beef).
Continue to add a bit of broth to the rice so that it does not stick to the pan while it is cooking slowly.
Observe the cooking time for the rice that you use.
While the rice is cooking, in a separate non-stick frying pan, put a bit of olive oil and cook some other mushrooms that you will add to the rice when it is finally cooked, along with some chopped parsley (if you like) and plenty of grated parmesan cheese.