Preheat the oven to 200 C / 400 F. Line a sheet pan (33 x 46 cm / 13 x 18 inch) with parchment paper.

Unroll the puff pastry on a lightly floured surface and cut into 6 squares, about 10 cm / 4 inches each. Using a paring knife, score a 1 cm border around each square without cutting all the way through.

Transfer the squares to the sheet pan, prick the centers a few times with a fork, and chill in the fridge for 15 minutes while you make the filling.

Heat the butter and olive oil in a 12-inch / 30 cm skillet over medium heat. Add the shallot and cook for 2 minutes until softened.

Stir in the garlic and cook for 30 seconds until fragrant. Add the mushrooms and raise the heat to medium-high.

Cook the mushrooms for 8 to 10 minutes, stirring occasionally, until they release their liquid and turn deep golden brown at the edges. This is the step people rush. If the pan still looks wet, keep going, the mushrooms need to brown, not steam.

Add the thyme and sherry, scraping up any browned bits from the pan, and cook for 1 minute until the liquid mostly evaporates. Stir in the cream, salt, and pepper, and simmer for 1 minute until slightly thickened. Remove from heat and cool for 5 minutes.

Brush the border of each pastry square with the egg wash. Bake for 10 minutes at 200 C / 400 F until pale gold and puffed.

Remove the pan from the oven and gently press down the center of each pastry with the back of a spoon to make room for the filling.

Spoon the mushroom mixture into the centers and top each tartlet with grated Gruyère.

Return to the oven and bake for 10 to 12 minutes, until the cheese is melted and the pastry edges are deep golden brown.

Cool on the pan for 5 minutes, scatter with extra thyme leaves, and serve warm.