1)  Sauté the chicken and vegetables first. Heat 3 tablespoons cooking oil in a large skillet or pan over medium-high heat. Add the diced chicken, sliced leek and rosemary. Fry for 4 minutes while stirring occasionally. Then add the sliced mushrooms and white wine. Sauté for another 3 minutes.

2)  Combine the cornstarch and 2 tablespoons of chicken stock in a small mixing bowl. Stir fast until no lumps remain. Add the remaining chicken stock, crème fraîche and cornstarch to the pan. Simmer for 5 to 10 minutes until thickened. Add the parsley and season with salt and pepper to taste.

3)  Preheat the oven to 350°F/180°C/gas mark 4. Grease the bottom and sides of an ovenproof pie dish with 1 tablespoon of cooking oil.

4)  Unroll the puff pastry sheet and cut out slightly larger than the pie dish. Spoon the filling into the dish and spread out into an even layer. Cover with puff pastry and tuck the edges into the sides of the dish. Using a sharp knife make 3 cuts into the puff pastry so the steam can escape while baking.

5)  Mix together the cream and egg yolk. Brush the puff pastry with the egg glaze and bake for 25 minutes until the pastry looks puffed and golden. Remove from the oven and let cool for 10 minutes before serving.