Melt butter in a pressure cooker over medium low heat.
Add onion and cook 5 minutes or until soft.
Add mushrooms and cook 2 minutes.
Add rice and cook another 2 minutes.
Stir in broth and mix well.
Lock the lid in place.
Cook on high heat to bring to high pressure.
Reduce heat just enough to maintain high pressure.
Cook 7 minutes.
Turn off heat.
Allow pressure to come down naturally, about 10 minutes.
Tip lid away from your face and remove.
Stir in Parmesan cheese.
Season with pepper.