Heat oven to 375 degrees 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.
Serves about 15 as hors d'oeuvres.
Mushrooms may be stuffed and refrigerated ahead of time, then baked as needed.
This recipe can be adjusted, using the same proportions, to make as many mushrooms
as you want.