Melt the butter in a small saucepan over a medium heat. Add the flour and cook for 1 minute, stirring constantly.
Gradually add the stock, stirring until well combined. Cook for about 5 minutes, stirring often until the sauce boils and thickens. Season with sea salt and freshly ground black pepper.
Stir in the mustard, lemon juice and Tabasco sauce and leave to simmer gently over a very low heat.
In another pan, melt a knob of butter and gently fry the onion, mushrooms and drained morels for about 5 minutes until soft. Add the wine, bring to the boil, then add this mixture to the simmering sauce, stirring for a couple of minutes.
Stir the chicken, tarragon and parsley into the sauce, then remove the pan from the heat and allow the mixture to cool.
Preheat the oven to 200oC/400oF/Gas Mark 6.
Spoon the chicken mixture into a pie dish and top with a puff pastry lid, making sure it’s sealed round the edges.
Brush the pastry with beaten egg and bake for about 30 minutes or until golden. Serve with buttered savoy cabbage.