1 sheet frozen puff pastry, thawed and cut into six 3-inch rounds
1 tablespoon extra-virgin olive oil, plus more for muffin pan
1 large shallot, diced small
1 pound cremini mushrooms, sliced
Coarse salt and ground pepper
2 teaspoons fresh thyme leaves, plus more for serving
3/4 cup grated Gruyere cheese (2 ounces)

1 sheet frozen puff pastry, thawed and cut into six 3-inch rounds 1 tablespoon extra-virgin olive oil, plus more for muffin pan 1 large shallot, diced small 1 pound cremini mushrooms, sliced 2 teaspoons fresh thyme leaves, plus more for serving 3/4 cup grated Gruyere cheese (2 ounces)

Preheat oven to 375 degrees. Place puff pastry rounds in a single layer on a parchment-lined baking sheet and refrigerate while mushrooms cook. In a large skillet, heat oil over medium-high. Add shallot and cook, stirring, until soft, 3 minutes. Add mushrooms and cook, stirring, until soft and browned, 10 minutes. Season with salt and pepper. Stir in thyme and remove skillet from heat.

Lightly oil 6 jumbo muffin cups. Divide mushroom mixture and cheese among cups, then top each with a chilled pastry round. Bake until pastry is golden brown and puffed, about 25 minutes, rotating pan halfway through.

Run a small knife around cups to loosen tartlets. Place a rimmed baking sheet or large plate over pan and invert to release tartlets. Sprinkle with more thyme. Serve warm.