Let me tell you my friend, these are delicious, and very simple! I make them with the small mushrooms so they are pretty much bite size, and I can eat many more than I should! Here is the recipe I use: http://www.ehow.com/how_14630_make-stuffed-mushroom.html. I've made them for a game night at my house with my friends and for Christmas Eve with my family and received great feedback both times.
Take the stem off the mushrooms (save the stems!)
Combine cream cheese with shredded cheese, chives, and green onions.
This is where what I do differs from the recipe a little, I also chop up the mushroom stems into little pieces and mix that in with those ingredients as well
Fill each mushroom cap with the mixture
Cover each mushroom with shredded cheese
Bake in the oven at 350 degrees until the cheese is melted