In a medium sauce pan, heat olive oil over medium heat. Saute mushrooms until soft, 3-5 minutes. Remove from pan.
Add the water to the pan and bring to a boil. Sprinkle the polenta over the top and stir in immediately. Do not dump it in, it will clump and you will be sad.
Reduce heat to low and simmer until polenta is thick, about 10 minutes.
Stir in butter, Parmesan cheese and mushrooms. Serve.