In a large skillet, melt the butter over medium heat.
Add the mushrooms and onion. Sauté until the onion is soft and translucent.
Add one tablespoon of balsamic vinegar, stir, and continue cooking until the liquid has evaporated and the onions begin to caramelize.
Add the garlic and thyme, stirring briefly.
Add the chopped chestnuts, the fresh spinach, and the remaining balsamic vinegar. Stir until the spinach wilts and the mixture is well combined. Season with salt and pepper.
Transfer the mixture into a large bowl. Stir in the breadcrumbs and walnuts until the filling is moist but holds together.
Cover and chill in the fridge for about 30 minutes.
Preheat your oven to 375 °F (190 °C).
Line a baking sheet with parchment paper.
Roll out the puff pastry into a large rectangle. Place the filling in the centre and shape it into a long log. Fold the pastry over the filling, sealing the edges.
Fold the pastry over the filling, sealing the edges.
Brush the top with the beaten egg to achieve a golden, shiny crust. Use a sharp knife to make a diamond-pattern or small incisions on top for decoration.
Bake for about 30 minutes, or until the pastry is golden brown and cooked through. Let cool slightly before slicing. Serve warm—cranberry sauce on the side brings a lovely sweet-tart balance.
Cool&Simple Culinary Team
Behind every Cool&Simple recipe is our love for great ingredients and easy-to-make ideas.
