Preheat your oven to 400°F (200°C).
Unroll the puff pastry sheet onto a baking tray lined with parchment paper.
Heat olive oil in a skillet over medium heat. Add mushrooms and garlic, and sauté until mushrooms are golden brown.
Season the mushroom mixture with salt, pepper, and thyme.
Spread the mushroom mixture over the puff pastry, leaving a border around the edges.
Sprinkle gruyere cheese over the mushrooms.
Bake in the preheated oven for 25 to 30 minutes, or until the pastry is puffy and golden.
Serve warm, garnished with additional thyme if desired.