This is a pretty easy recipe I threw together that I like to serve as a side dish.  I usually only eat 2 at a time because they are very rich and filling.

Low Carb Stuffed Mushrooms
8 average size button mushrooms (the 8 oz packages usually have 8 mushrooms)
3 slices bacon
1/4 medium onion, finely chopped (this worked out to be about 1 oz of onion)
3 oz cream cheese (I used the garden vegetable flavor), softened.
1 oz shredded cheddar cheese
Garlic salt

Preheat oven to 350 degrees.
1) Gently wipe off any dirt from the mushrooms using a paper towel or mushroom brush (do not wash!).
2) Remove the stems from the mushrooms, leaving a hollow in the mushrooms to hold the stuffing.
3) Finely chop the mushroom stems.
4) Fry the bacon in a skillet until crispy.  Remove the bacon, wrap in a couple of paper towels and allow to cool.
5) Toss the onion and mushroom stems into the bacon grease.  Sprinkle on a couple dashes of garlic salt and saute until the onion is clear.
6) Crumble or chop the cooked bacon pieces.  I like the pieces to be fairly small so I don't end up with huge chunks of bacon in my filling.
7) Mix the bacon pieces, sauteed onions and mushroom stems, cream cheese and shredded cheddar together in a bowl.
8) Stuff each mushroom cavity with a generous spoonful of the cheese mixture. 
9) Bake in the oven for about 30 minutes, until the mushrooms release their juices.
10) Finish off for a few minutes under the broiler to brown the tops of the mushrooms.

Experiment with different flavors of the cheeses.  I think next time I might try spinach and artichoke cream cheese with parmesan cheese in place of the cheddar.

Nutrition Information for each mushroom (as calculated by DietPower):
Calories 69, Carboydrates 1.33g (Fiber .254 g, Sugars .522 g), Fat 6.11 g (Saturated 3.5 g, Polyunsaturated .344 g, Monounsaturated 1.94 g, Trans 0 g), Protein 3 g, Cholesterol 17.3 mg, Sodium 78.6 mg, Potassium 91mg