Step 1

Preheat the oven to 190C/ 374F.Line a baking tray with parchment paper.

Step 2

Heat 1 tablespoon of olive oil in a frying pan. Add the onions with 1/4 teaspoon of sea salt and cook over medium heat until soft. At least 15 minutes. 20 - 25 minutes if you have more time.Once soft add 1 tablespoon of balsamic vinegar and stir well until the liquid evaporates.Remove to a large mixing bowl and leave to cool.

Step 3

Heat 1 tablespoon of butter in a large sauté pan. Add the mushrooms and cook until they are soft and they have released their juices. Add 4 minced cloves of garlic and 1 tablespoon of chopped tarragon. Stir well and cook for a minute or two.Turn up the heat then add 1 tablespoon of red wine and stir well. Cook until all the liquid has evaporated.Crumble the chestnuts into the pan and mix well. Remove this mixture to the mixing bowl with the red onions and leave to cool.

Step 4

Heat 1/2 a tablespoon of olive oil in a pan, add the spinach and cook until wilted.Drain and squeeze as much liquid out of the spinach as you can. Transfer to a chopping board then roughly chop.Add the chopped spinach to the mixing bowl with the mushrooms.

Step 5

Add 3 tablespoons of fine breadcrumbs along with 1/4 teaspoon of salt and 1/4 teaspoon of freshly cracked black pepper to the mixing bowl. Mix everything together well.

Step 6

Roll out the puff pastry.Add the mushroom filling along the centre of the pastry and using clean hands shape into a sausage shape.Turn one side of the pastry over the mushroom mixture, brush the egg wash then turn the other side over. Use a fork to seal the pastry. Fold up the ends of the wellington and again use a fork to see them.Carefully turn the welling over so the seal is on the bottom. Using a very sharp knife score diamonds in your pastry. Start by scoring diagonal line in one direction along the whole wellington then repeat in the opposite direction.Brush the whole wellington in egg wash, remembering to do the sides too.Bake in the oven for 30 minutes or until the pastry is golden brown and the pastry is completely cooked.Remove from the oven and leave to cool slightly before transferring to a serving dish or board.