Prepare the hot water crust according to the recipe
Heat the oven to 180C or 350F
Line the pie dish or cake tin you are using with pastry (10 inch or 20 cm). I roll and fill the pie dish or cake tin leaving a bit of pastry overhanging the sides. Fill it with paper then beans or baking beads and bake at 180 C for around 15 minutes. Remove the paper and bake a further 5-10 minutes or till starting to colour. the pastry will cook more once the filling is in it too. Once done set it aside till the filling is ready
Put a medium pot on the heat and add the olive oil the onion, garlic, celery and oregano. Sweat it gently for 5 minutes or till soft add the chicken and break up a bit while stirring until it starts to colour. Add the flour and stir through and heat well before adding ¾ of the stock and the spinach. Keep stirring and breaking down the spinach and the sauce thickens. Season well tasting as you go. Stand aside while you finish the mash.
While the chicken is cooking put the potatoes on to steam. Prepare the feta mixture by breaking it up into the milk. warm the milk in a saucepan slowly or use the microwave till just warm- don't boil, then break up the feta with a fork and mix through the milk. Once the potatoes are riced or mashed add the butter and salt. Mix well then start to add the milk. Mixing well and continue to add till it's all gone. Taste for salt and pepper. Add if needed. Finally break the yolk with a fork and mix in 1 Tablespoon water then whisk it briskly through the potato till incorporated.
Pour the filling into the pastry
Spoon or pipe the potato onto the top of the pie.
Bake for 30-40 minutes or until golden and bubbling. Test the internal temperature is around 80 C or by poking a knife into the centre of the pie after 30 minutes, feel the knife to test the temperature of the centre of the pie. Remember the filling is already cooked.