1. First prepare the pastry by adding the flour, baking powder, salt, butter and egg yolk to a food processor. Use the cutting blades and pulse function to process until the mixture looks like fine breadcrumbs. Alternatively cube the butter and rub it into the dry ingredients, adding the egg yolk afterwards.
2. Add the water a bit at a time until the dough comes together in a ball. Wrap this in cling film and put in the fridge for an hour to chill.
3. Turn on the oven to 180˚C/350˚F to pre-heat.
4. Dice the onion, and cube the chicken and ham/gammon, making the ham pieces approximately half the size of the chicken pieces.
5. Fry the onion until soft, then add chicken and brown.
6. Add the ham or gammon, and cook, stirring for approximately 10 minutes until the chicken is cooked through.
7. Add the mustard to the chicken and ham, and stir.
8. Take off the heat and stir in the sour cream and chopped tarragon. Return to the heat.
9. Mix the cornflour with a little cold water, add to the pan and bubble for 2-3 minutes until thickened. Turn off the heat and allow to cool.
11. Grease your pie dish, roll out approximately 2/3 of your chilled pastry on a floured surface until it is large enough to cover the base and sides of the dish. Use the rolling pin to transfer it to the dish and gently press it down inside against the base and sides, then prick the base several times with a fork.
12. Trim the excess pastry, line the pie with greaseproof paper, and cover the base with rice or dried beans. Bake in the oven for 10 minutes, then discard the paper and beans, and allow the pie base to cool.
13. Add the filling to the pie base.
14. Roll out the rest of the pastry to a size large enough to cover the top of your dish. Beat the egg and brush the rim of your pie base. Use your rolling pin to pick up your pie lid and drape it over the top of the pie. Trim off any excess pastry. (Leftover tip: Use up the trimmed pastry to make jam tarts).
14. Crimp the edges with a fork, cut a small cross into the centre of the lid to allow steam to vent and brush the top of the pie with beaten egg.
15. Put in the oven and bake for 30-35 minutes until golden brown.
