Preheat your oven to 375F and move the rack to the middle position.
Microwave the cream cheese in 20-30 second intervals until's very soft.
Remove the mushroom stems and chop the stems up into small pieces. You may want to trim off the very tip of the stems and discard them as they can be woody.
Add the butter to a skillet over medium heat. Once melted, sauté the onions with the chopped mushroom stems until the water cooks off and they're lightly browned (about 7-10 minutes).
Stir in the garlic and cook for about 30 seconds.
Take the skillet off the heat. Stir in the cream cheese. Once it's been incorporated, stir in the parmesan, thyme, parsley, and breadcrumbs. Season with some salt & pepper to taste.
Using a small spoon and your hands, stuff the mushrooms with the cream cheese mixture, packing it fairly tightly. Place them on a baking sheet that's been coated with cooking spray (line with foil first for easy clean-up if desired).
Bake, uncovered, for 20-25 minutes or until the mushrooms are cooked through and golden on top (if the mushrooms are really large you may need a bit longer).