Preheat oven to 400.

Remove stems from mushrooms, and gently remove some of the gills with a spoon, being careful not to break the edge of the mushroom.

Coat baking pan with oil or nonstick cooking spray.

Place mushrooms on pan gill side down; bake for 5 minutes

Remove from oven, and flip mushrooms.

Fill each mushroom with parmesan.

Return to oven for 5-10 minutes, until the mushrooms are tender and the cheese is fully melted.

Season with salt and pepper.