Easy to make, beautiful to look at and delicious to eat, this rustic galette uses two different kinds of mushrooms for varied texture, lots of parmesan cheese and is all wrapped in buttery and golden puff pastry. Use store-bought pastry, or make your own rough puff if you prefer.
Preheat oven to 200°C/400°F. Using a food processor (or a sharp chef's knife and some patience) pulse the button mushrooms until they resemble a paste. You may need to work in batches. Transfer to a large mixing bowl. Add the leeks, garlic, rosemary, thyme and ¼ tsp of salt. Stir until well combined.
Add the butter and 1 tbsp of olive oil to a large skillet and set over medium heat. Once melted and lightly foamy, add the mushroom mixture. Cook, stirring frequently, until the mushrooms release most of their moisture and begin to brown - about 10 minutes.
Deglaze with the wine and cook until reduced and the strong alcohol smell has dissipated - about 2-3 minutes. Off heat, stir in the parmesan cheese.
On a lightly floured surface, roll the puff pastry in a circle until it is about 5mm (⅛-¼in) thick and reaches about 30cm (12in) in diameter. Transfer to a parchment-lined baking sheet.
Heap the filling into the centre of the pastry. Spread it out so that there is about 5cm (2in) of overhang around the edges. Arrange the torn oyster mushrooms over the filling.
Fold the edges of the pastry over the filling. Brush the exterior of the pastry with the beaten egg and then top with black pepper, flaky salt and a bit more parmesan cheese.
Move to the oven and bake until deeply golden and flaky - about 20 minutes. Allow to cool for 5 minutes before slicing and serving.