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.
Add the sausage meat to a skillet over medium-high heat and cook until just cooked through, breaking it up as you go along (about 5-7 minutes). Transfer to a plate.
Reduce the heat to medium and add the butter to the skillet. Once melted, sauté the onions until lightly browned (about 5-7 minutes).
Add the sausage meat back to the skillet, stir in the garlic, and cook for about a minute.
Take the skillet off the heat. Stir in the cream cheese. Once it's been incorporated, stir in the parmesan, thyme, parsley, and breadcrumbs.
Using a small spoon and your hands, stuff the mushrooms with the sausage mixture, packing it fairly tightly (there is quite a lot of filling!). Place them on a baking sheet that's been coated with cooking spray (line with foil first for easy clean-up).
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).