Pulse mushrooms in a food processor until finely chopped (texture of coarse ground meat). Do not puree them.

Heat 1 tbsp oil and 1 tbsp butter in a large wide pan over medium heat. Add finely chopped shallots and cook until soft (5 mins). Add minced garlic and cook for 1 more minute.

Add the chopped mushrooms and thyme leaves. Cook over medium-high heat until all the liquid has evaporated and the pan is dry (about 15–20 minutes).

Deglaze with sherry/wine (if using) and cook until dry again. Stir in the soy sauce.

Crumble the chestnuts into the pan (leave some chunks for texture) and stir in the breadcrumbs. Season generously with salt and pepper. Spread mixture onto a tray to cool completely.

In a clean pan, wilt the spinach with a splash of water. Place wilted spinach in a clean kitchen towel or cheesecloth and squeeze out every drop of liquid. Season and chop the spinach roughly.

Roll out the puff pastry on a piece of parchment paper to a rectangle (approx. 10x14 inches). Spread the dried spinach down the center of the pastry, leaving a 2-inch border on the sides. Spoon the cooled mushroom/chestnut mixture on top of the spinach.

Pack the mixture into a tight log shape with your hands. Lift the parchment paper to help roll the pastry over the filling. Overlap the edges and pinch to seal. Tuck the ends underneath.

Place the log seam-side down on a baking sheet. Using a sharp knife, lightly score a cross-hatch or diagonal pattern on top. Brush the entire surface with beaten egg.

Preheat oven to 400°F (200°C). Bake for 35–40 minutes until the pastry is deep golden brown and puffy. Let it stand for at least 10 minutes before slicing.