Preheat oven to 200 degrees celcius. Grease a baking tray with cooking spray. Remove stems from mushrooms and roughly chop the stems. Place mushroom caps on baking sheet.
In a medium frypan over medium heat, melt butter. Add chopped mushrooms stems and cook until most of the moisture is out. This usually takes around five minutes. Add garlic and cook for one minute until fragrant. Season with salt and pepper. Remove from heat and let cool slightly.
In a large bowl mix together mushroom stem mixture, prepared gluten-free stuffing, Parmesan, parsley, and thyme. Season with salt and pepper. Fill mushroom caps with filling and sprinkle with more parmesan.
Bake in oven until mushrooms are soft and the tops are golden brown. This will take around 20 minutes.