Preheat oven to broil at 500º F. Place mushroom caps in a skillet. Spoon in about 1 tablespoon of pizza sauce per mushroom cap. Top with pepperoni slices and cheese. Place skillet into oven for 8-10 minutes, until mushroom has softened and cheese has begun to melt and bubble slightly. Do not overbake. If mushroom does not soften on the broil setting, remove skillet from the oven and place on the stove for about 3-5 minutes.
