Sauté mushrooms 5 minutes. Place puff pastry on baking paper, fold a 1 cm / 0.4" border, brush edges with butter, top with Gruyère and mushrooms. Bake on the preheated tray for 30 minutes. Finish with fresh thyme, salt flakes and truffle oil.

Hot tray - Place a baking tray in the oven. Turn the oven on and preheat to 210°C / 410°F (190°C fan).

Cook the mushrooms - Heat the olive oil and butter in a frying pan over medium-high heat. Add mushrooms, salt and pepper. Cook for 5 minutes, stirring regularly, until soft. Transfer mushrooms to a colander over a bowl to drain in case of excess liquid.

Prepare the base - Place a sheet of baking paper / parchment paper on the bench. Put the puff pastry on top. Fold the edges inwards to create a 1 cm / 0.4″ border and brush them with melted butter.

Assemble - Spread the grated Gruyère evenly over the pastry. Top with the mushrooms, spreading them evenly too.

Bake - Lift the baking paper and tart onto the hot tray in the oven. Bake for 30 minutes, until the pastry is golden and crisp (sides and underneath).

Finish and serve - Sprinkle with fresh thyme leaves and salt flakes. Drizzle with truffle oil or a good virgin olive oil (optional but highly recommended). Cut and serve hot.