Preheat the oven to 180°C/ 350 F/ Gas 4. Meanwhile, heat the oil in a deep – sided frying pan and fry the onions and carrots for a few minutes until softened.
Add the cooked turkey to the onions and carrots and stir in the mustard and mushrooms. Cook for a further 3-4 minutes.
Make a little room at the base of the pan and add the flour. Stir the milk into the pan and simmer the mixture until thickened. Season well. Spoon into an ovenproof dish.
Roll out the pastry and cut 8 long strips. Brush the pastry with a little milk and twist each length. Arrange over the mixture in a criss–cross pattern. Bake for 40 minutes. Spoon the pie onto plates, garnish with parsley and serve with steamed vegetables of your choice.