For the pastry, rub the cold butter into the flour in a bowl until it resembles breadcrumbs.
Add the kefir or yoghurt, eggs and salt, then mix together well. Knead the dough, adding more flour if the pastry is still too wet – you are looking for a soft, but not particularly damp, dough.
Wrap in clingfilm and leave to rest and firm in the refrigerator for 15 minutes. Meanwhile, mix all the ingredients for the filling together. Preheat the oven to 180C/350F/gas 4. Grease a 24cm (9½-in) round shallow cake tin or pie dish.
Divide the pastry dough in half. Roll out one half on a lightly floured work surface and use it to line the greased tin or dish. Add the filling and spread it evenly over the base of the pastry case.
Roll out the other piece of dough, lay it over the top and pinch the edges together. Brush with the beaten egg yolk to glaze and prick it all over with a fork to allow the steam to escape.
Bake the pie for about 20–30 minutes, or until the pastry is golden and cooked through.