1. Preheat the oven to 400°F and line a sheet pan with parchment paper. Roll the puff pastry sheet into a 9x11 rectangle. Score a 1/2-inch border around the tart and poke some holes in the pastry using the tines of a fork. Place the puff pastry on the prepared sheet pan and refrigerate while you prepare the remaining ingredients.
2. Heat the olive oil and 1 tbsp of unsalted butter in a large skillet over medium high heat. Add the mushrooms, a big pinch of salt, and black pepper, to taste. Sauté the mushrooms until they’re golden-brown and slightly softened but not cooked all the way through. Add the garlic and cook for 30 seconds. Then, transfer the mushrooms to a bowl to cool slightly.
3. Next, whisk the egg with 1 tbsp of water to make the egg wash. Set aside. Using a mandoline, slice the Yukon gold potatoes as thin as possible.
4. Remove the sheet of puff pastry from the refrigerator and brush the edges with the egg wash. Layer the potatoes in an overlapping shingle pattern on the puff pastry, inside the border. Melt the remaining 1/2 tbsp of butter and brush the potatoes with the melted butter. Season generously with salt and pepper.
5. Layer the mushrooms over the potatoes. Bake the tart for 25 minutes until the potatoes are tender and the puff pastry is golden-brown.
6. Remove from the oven and place the slices of Petite Truffle in rows on top of the tart, then bake for another 1-2 minutes, just until the cheese is melty. Garnish with minced chives. Slice and serve.