Preheat oven to 375° F. Lightly grease a large cookie sheet; set aside. (If desired, line cookie sheet with aluminum foil to save on cleanup.)
Wash mushrooms thoroughly; dry and remove stems, hollowing out a hole where the stem was attached to the mushroom. Set aside.
Mix together cheeses, mayonnaise and bacon (if using). Pack the center of each mushroom with a mound of the cheese mixture. Arrange mushrooms on cookie sheet. Bake until cheese is browned and mushrooms are shiny, about 20 minutes.
Mushrooms may be stuffed and refrigerated ahead of time, then baked as needed.Makes 15 Source: