Slice the shallots and mushrooms into thin pieces; chop the garlic finely.

In a medium skillet, melt the butter over medium heat. Add sliced shallots and thyme; sauté for 3-4 minutes.

Increase heat and add mushrooms; cook for 5-7 minutes until browned. Add balsamic vinegar and let it evaporate.

Combine sautéed mixture in a bowl, season with salt and pepper, and let cool slightly.

Preheat oven to 375°F (190°C). Roll out puff pastry and score a border; poke holes in the base.

Sprinkle Gruyere cheese over the pastry base, then layer with the mushroom mixture.

Bake for 20-25 minutes until golden brown and cheese is bubbling.

Allow to cool slightly before serving.