Roll out the puff pastry on a lightly floured surface into two rectangles, 12" x 9" each and set aside, covered up in the fridge until required.
To make the filling, heat the oil in a large pan and fry the onion with half the garlic for at least 20 minutes or until coloured deep brown and golden.
Remove the pan and set the onion and garlic mixture aside.
Add the mushrooms to the same pan with the rest of the garlic and half the tarragon, and cook on a fairly high heat.
Halfway through cooking, add the soy sauce and continue until the mushrooms are cooked through. Season with salt and pepper, then set the mushrooms aside, reserving all the mushroom liquor.
In a food processor, blend the cashews with the mushroom liquor to a fine smooth puree adding a little water to help.
Remove from the blender, then blend the onions, then the mushrooms, until each is perfectly smooth.
Mix all the blended ingredients together in a bowl, adding the breadcrumbs, ground almonds and remaining tarragon. The mixture should gently hold its shape when formed in the hand.
Preheat the oven 220°C / 425°F / gas mark 7.
Remove the pastry from the fridge and place on a lightly floured surface. Separate the two rectangles and place half the filling onto one of the pastry pieces, shaping it with your hands to make a long rectangular shape and leaving 6" at either side of the pastry.
With a sharp knife, make diagonal cuts at angle from the left hand corner of the pastry towards the filling and working your way down the length of the pastry. Repeat on the other side, this time starting from the top right hand corner and cutting towards the pastry. Each strip should be half an inch wide.
Fold the end pieces in first. The draw a strip over from the left, then the right, crossing each other the opposite and repeating all the way down the pastry.
Glaze with a beaten egg.
Place on a floured tray and bake in the oven for 35-45 minutes until golden.
Allow to cool for a few minutes before attempting to serve.