Preheat the oven to 180°C. Lightly grease a baking tray.
In a large skillet cook the bacon over medium heat until crispy. While the bacon cooks, use paper towel to wipe the mushrooms clean. Remove the stems and coarsely chop them. Place the caps on the baking sheet.
Once the bacon is crispy, drain it on a paper towel and then crumble it. Keep 2 tbps of the bacon grease in a small bowl and throw the rest away.
Return the skillet with the remaining bacon grease to the heat. Add the chopped mushroom stems along with the onion and garlic and cook until they are tender. Transfer the mixture to a large bowl and add the crumbled bacon and cream cheese.
Spoon about 1 tablespoon of the filling into each of the mushroom caps. Bake for 18 to 20 minutes or until the filling is heated through and golden brown on the top.