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Preheat your oven to 210°C / 410°F (190°C fan) with a baking tray inside to heat up.

In a large pan, heat 1 tablespoon of extra virgin olive oil and 2 tablespoons (30g) of unsalted butter over medium-high heat. Add the sliced Swiss brown mushrooms, 1/2 teaspoon of cooking salt (or kosher salt, half for table salt), and 1/4 teaspoon of black pepper. Cook for 5 minutes, stirring occasionally, until the mushrooms are soft and browned. If there are excessive juices, drain them using a colander.

Lay the barely thawed butter puff pastry sheet (25cm / 10″ square) on a piece of baking paper. Fold the edges of the pastry inwards by about 1cm / 0.4” to form a border. Brush the folded border and the main surface of the pastry inside the border with the 2 teaspoons (10g) of melted unsalted butter.

Evenly sprinkle the 1 1/2 cups (150g) of grated Gruyère cheese over the pastry, inside the buttered border. Then, spread the cooked and drained mushrooms evenly on top of the Gruyère cheese layer.

Carefully transfer the assembled tart, still on the baking paper, onto the preheated hot tray in the oven. Bake for 30 minutes, or until the pastry is golden brown and crisp underneath.

Remove the tart from the oven and transfer it to a cutting board. Finish by sprinkling fresh thyme leaves and salt flakes over the baked tart. Drizzle with truffle oil or good extra virgin olive oil, if desired. Slice into desired portions and serve immediately.


Preheat your oven to 210°C / 410°F (190°C fan) with a baking tray inside to heat up.

In a large pan, heat 1 tablespoon of extra virgin olive oil and 2 tablespoons (30g) of unsalted butter over medium-high heat. Add the sliced Swiss brown mushrooms, 1/2 teaspoon of cooking salt (or kosher salt, half for table salt), and 1/4 teaspoon of black pepper. Cook for 5 minutes, stirring occasionally, until the mushrooms are soft and browned. If there are excessive juices, drain them using a colander.

Lay the barely thawed butter puff pastry sheet (25cm / 10″ square) on a piece of baking paper. Fold the edges of the pastry inwards by about 1cm / 0.4” to form a border. Brush the folded border and the main surface of the pastry inside the border with the 2 teaspoons (10g) of melted unsalted butter.

Evenly sprinkle the 1 1/2 cups (150g) of grated Gruyère cheese over the pastry, inside the buttered border. Then, spread the cooked and drained mushrooms evenly on top of the Gruyère cheese layer.

Carefully transfer the assembled tart, still on the baking paper, onto the preheated hot tray in the oven. Bake for 30 minutes, or until the pastry is golden brown and crisp underneath.

Remove the tart from the oven and transfer it to a cutting board. Finish by sprinkling fresh thyme leaves and salt flakes over the baked tart. Drizzle with truffle oil or good extra virgin olive oil, if desired. Slice into desired portions and serve immediately.
