1. Put the stock, carrots, and parsnips in a large saucepan over high heat and bring to a boil. Cook for 5 minutes or until the vegetables are crisp-tender. Strain in a sieve over a bowl, and reserve the stock and vegetables.

2. Melt the butter in another saucepan over medium heat. Add the onion and celery, and cook, stirring often, for 5 minutes or until softened. Sprinkle in the flour and stir for 1 minute.

3. Gradually stir in the reserved stock and bring to a boil, stirring. Reduce the heat and simmer for 2 minutes. Stir in the fava beans and mustard, and season with salt and pepper. Remove from the heat and cool until tepid. Stir in the chicken, cream, and reserved vegetables. Pour into a 9.5in (24cm) deep-dish pie pan.

4. Preheat the oven to 400°F (200°C). Roll out the pastry on a lightly floured surface until about 1/8 in (3mm) thick. Cut out a round of dough slightly larger than the dish. Brush the dish rim with water, then position the pastry over the filling, and fold over the excess pastry so the dough is double-thick around the circumference of the pan. Crimp the pastry, glaze the top with the some beaten egg, and cut a small hole in the top.

5. Place the pie on a baking sheet. Bake for 25 minutes or until the pastry is puffed and golden, and the filling is hot. Let cool for a few minutes, then serve.

Variation:

Chicken and Leek Pie: Replace the onion with chopped leek for a subtle flavor change and add or substitute other vegetables, such as green beans or corn.