Heat the oven to 180C or 350F
Line and blind bake the pastry crust base and set aside. 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
Put a medium pot on the heat and add 4 tablespoons of olive oil and add 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 3/4 of the stock and the spinach. ( if using fresh spinach add last). 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. Break the feta up into the milk and warm ( I use the microwave till just warm- don't boil),then break up the and mix through the milk with a fork. Once the potatoes are riced or mashed add the butter and salt. Mix well then start to add the milk. Mixing and continue to add till it's all gone. Finally break the yolk with a fork and mix in 1 Tablespoon water then whisk it briskly through the potato till incorporated. taste for salt and pepper. add if needed.
Pour the filling into the pie.
Spoon or pipe the potato onto the top of the pie.
Bake for 30-40 minutes or until golden and bubbling. Test for doneness 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.