Heat olive oil in large skillet and cook mushrooms over medium heat until golden brown. Season and keep warm.In a medium saucepan, bring stock or water to a boil. Stir in cornmeal. Reduce heat to a simmer and stir occasionally to prevent lumps. Cook for approximately 30 minutes. Taste the polenta, it should be thick and smooth without a coarse texture. Cook over low heat until the texture is correct. Stir in butter, cheese and salt and pepper. Serve hot as a side or main dish.