Preheat the oven to 375˚F. Line a baking sheet with parchment paper.

Heat a pan over medium heat. Add in the butter and allow it to melt. Mix in the shallots and thyme. Saute until the shallots turn translucent. Add in the garlic and cook until frgrant, about 1 minute. Mix in the Mushrooms and balsamic vinegar. Cook until all of the moisture from the mushrooms evapoates. Season with salt and pepper to taste. Remove the mushrooms from the pan and set it aisde.

Use a knife to score around the edges of the puff pastry sheet, leaving a 1-inch border. Prick the center portion of the dough with a fork.

Dollop spoonfuls of the ricotta around the puff pastry. Distribute the mushrooms onto the puff pastry. Sprinkle gruyere cheese on top. Bruh the border of the puff pastry with the egg. Bake it in the oven for 20-25 minutes until the edges are golden brown and the cheese has melted. Remove the tart from the oven and allow it to sit for a few minutes before slicing.