Parchment paper, for baking, and a pinch of flour or cornmeal to dust the work surface
Instructions
Preheat your oven to 400°F (200°C). Line a baking sheet with parchment for easy cleanup and to prevent sticking.
Roll the puff pastry into a roughly 12-inch circle on a lightly floured surface. Transfer to the prepared baking sheet. Chill while you prepare the filling to keep the crust crisp.
In a large skillet, heat the olive oil and butter over medium-high heat. Add the onion (or shallot) and a pinch of salt; cook 3–4 minutes until translucent and slightly caramelized.
Stir in thyme and Dijon mustard (if using). Pour in the heavy cream and simmer 1–2 minutes until the mixture is lightly thickened and glossy. Taste and adjust seasoning.
Spread a thin layer of the mushroom filling in the center of the pastry, leaving a 1–1 1/2 inch border. Scatter the grated cheese over the filling.
Fold the pastry edge over the filling, pleating as you go to create a rustic circle. Brush the exposed crust with the beaten egg for a shiny, golden finish.
Bake 25–30 minutes, or until the crust is deeply golden and the filling is bubbling at the center. If the edges brown too quickly, tent with foil and continue baking until done. Let cool for 10 minutes before slicing and serving.
