Step 1

Drizzle the portobello mushrooms with 1 tbsp of the olive oil and season lightly. Heat a non-stick frying pan over a high heat and brown the mushrooms on either side for 2-3 minutes, then remove onto a plate.

Step 2

Heat the remaining olive oil in the same frying pan and add the onion and chestnut mushrooms over a medium-high heat with a big pinch of salt. Cook for 10 minutes until the liquid has evaporated from the mushrooms and they are caramelising. Add the garlic and thyme, and cook for another minute. Carefully tip in the brandy and bubble for a minute, taking care as it may flame. Reduce the heat and add the spinach and wilt for a minute before stirring in the cream, mustard and lemon juice, then bubble until well thickened. Season.

Step 3

Roll the pastry on a lightly floured worksurface to a 35cm x 35cm square, then cut into four equal squares. Transfer two to a baking tray lined with baking paper and brush the tops with a little of the mustard, then top with the half the creamy mushrooms. Pat the portobello mushrooms dry with kitchen paper and put on top, cup-side up. Spoon the remaining creamy mushrooms into the cup and around the portobello mushroom, leaving a 3cm gap around the edge. Brush this edge with beaten egg, then lay over the other piece of pastry. Use a small knife to trim into a rough circle, then use a fork to crimp the edges. Use the back of the knife to lightly score the pastry in a spiral pattern. Brush with beaten egg and chill for at least 30 minutes or until needed.

Step 4

Heat the oven to 200C/fan 180C/gas Brush the pithiviers with more beaten egg, sprinkle with thyme leaves and bake for 25-30 minutes or until deep golden.