Saute onion, garlic, and mushrooms in 2 tbsp of butter over medium heat.
Season with salt and pepper.
Saute for 20 minutes or until tender.
Add balsamic vinegar and cook for 2-3 additional minutes, until thickened.
Slightly roll out puff pastry sheet and cut into 9 squares.
Score a 1/2 inch "crust" on the outer edge of each puff pastry. Dock the middle with a fork.
Lightly grease a baking sheet and place puff pastry squares on it.
Sprinkle with parmesan.
Fill middle of puff pastry with mushroom filling.
Top with boursin or goat cheese crumbles.
Sprinkle with more parmesan.
Refrigerate until ready to bake.
Bake at 425 for 15-20 minutes until golden brown.
Garnish with thyme sprigs.