Thinly slice the onion, mushrooms, and garlic. Add to the pan and sauté gently for 10-20 minutes, stirring occasionally, until aromatic and caramelized. Season with salt and pepper to taste.
Preheat the oven to 425°F and line a baking sheet with parchment paper (this recipe makes 12 tarts so you'll probably need 2 baking sheets).
Cut each puff pastry sheet into 6 equal rectangles and arrange on your baking sheet.
If desired, lightly brush the edges of the pastry with egg wash.
Spread the shredded gruyere over the top of the pastry.
Then add the caramelized mushrooms and onions over top of the cheese and sprinkle with fresh thyme.
Bake at 425°F for 15-20 minutes or until the pastry is golden brown and the cheese is melted and bubbly.
Serve the mushroom puff pasty tarts warm and enjoy!!
Notes
These tarts are best served the same day, but will keep in an airtight container in the fridge for up to 3 days.