In a large saucepan, heat 1½ tablespoons of oil and cook the mushrooms until soft, about 3 minutes
Put the mushrooms and the liquid in a bowl and reserve.
Add the remaining 1 tablespoons oil to saute pan, and cook the onions for 1 minute.
Stir in the rice, coating it with oil, and toast for about 2 minutes.
When the rice has taken on a pale, golden color, add 1/2 cup broth to the rice, and stir until the broth is absorbed.
Continue adding broth 1/2 cup at a time, stirring continuously, until the liquid is absorbed and the rice is al dente. Taste test the rice - it should be soft with just a touch of bite in the center.
Remove from heat and stir in mushrooms with their liquid, butter, and parmesan.
Stir in parsley. Season with salt and pepper to taste.