Preheat your oven to 190°C (374°F) and line a baking tray with parchment paper.

In a frying pan, heat 1 tablespoon of olive oil over medium heat. Add the finely chopped red onions and ¼ teaspoon of sea salt, sauté until soft and caramelized, about 15-25 minutes.

Stir in 1 tablespoon of balsamic vinegar and cook until evaporated, then set aside to cool.

In another pan, heat 1 tablespoon of butter over medium heat. Add 280g of chopped chestnut mushrooms and cook for about 8-10 minutes until they are tender.

Stir in 4 minced garlic cloves and 1 tablespoon of chopped tarragon, and 1 tablespoon of red wine if using; cook until the liquid disappears, then mix with the cooled onion mixture.

In the same pan, wilt 100g of baby spinach in a small drizzle of olive oil over medium heat. Remove from heat, drain excess liquid, and chop.

Add the chopped spinach to the mushroom filling along with 3 tablespoons of fine breadcrumbs and a pinch of salt and pepper, stirring well to combine all ingredients.

Roll out a sheet of puff pastry on a lightly floured surface. Place the filling in the center and fold the pastry over, sealing the edges tightly. Score the top with a knife to allow steam to escape.

Brush the top of your Mushroom Wellington with the beaten egg. Place it on the baking tray and bake for about 30 minutes, or until the pastry is puffed and golden brown.