Put the spinach in a colander and place in the sink. Slowly pour boiling water from a kettle over the spinach to wilt, then leave to drain and cool. Squeeze out the liquid from the spinach, roughly chop and set aside.

Remove the stalks from the mushrooms and finely chop or pulse in a food processor then set aside. Working in batches, heat tablespoons of the olive oil in a large frying pan and sizzle the Portobellos for 4-5 mins on each side until cooked through. Set aside in a dish to catch any juices.

Put the same pan back on the heat with the remaining oil. Fry the shallots for a few minutes until soft and golden, then add the garlic and cook for 1 min. Add the chopped mushroom stalks and thyme, season with salt and pepper, then cook until soft and any liquid has evaporated. Stir the spinach and miso into the pan along with any mushroom cooking juices and the lemon juice, then cook for 4-5 mins until you have a loose, dry stuffing. Season with pepper and set aside to cool.

Mix all the ingredients for the glaze except the sesame seeds together. Roll a block of pastry out to the thickness of a £1 coin on a lightly floured surface. Using a saucer, cut out four circles about 5cm wider than the Portobellos (for the bottoms of the pithiviers) and place them on a flat baking tray lined with baking parchment. Top each one with a whole mushroom, then fill each mushroom with a quarter of the spinach mix. Top each with another mushroom, rounded-side up. Roll out the second block of pastry and cut out four more circles, roughly 5cm wider than the bases, for the tops of the pithiviers.

Brush the border around the mushrooms with the glaze, then gently stretch the larger circle over the mushrooms, trying not to trap any air, as if you’re making a giant ravioli. Press the edges together and trim, then crimp or press with a fork to seal. Score lines on the pastry with the back of a knife, if you like. The pithiviers can be chilled for two days, ready to bake, or frozen for a month (if the pastry wasn’t already frozen).

Heat the oven to 220C/200C fan/gas 7. Brush the pithiviers all over with the glaze and scatter with sesame seeds, if you like. Bake for 20 mins, then drop the temperature to 200C/180C fan/gas 6 and bake for another 20 mins until deep golden. Leave to cool for a few minutes before serving. To be fancy, neatly cut in two and serve as two halves to show the cross-section of stuffed mushrooms.