Preheat oven to 350F degrees. Using scissors or a sharp knife, cut raw bacon into small pieces. In a large saute pan, over medium heat, cook bacon until nice and crispy. While bacon is cooking, remove mushroom stems from caps and chop stems into small pieces. When bacon is done, remove from pan and set aside; reserve 2 tablespoons of bacon grease and pour the rest from pan. Saute onion until soft about 5 minutes, scraping up any brown bits on bottom of pan. Add chopped mushrooms stems and garlic and cook a few minutes longer. Reduce heat to low. Add cream cheese and parmesan cheese and stir until cheeses are melted. Add reserved chopped bacon and season to taste with salt and pepper. (Mixture can be made, cooled, and stored, covered, in the fridge for up to two days.) Remove mixture from heat and stuff each mushroom cap generously with mixture. Bake at 350F for about 20 minutes or until mushrooms are soft and filling is nice and hot.