Boil the rice in enough water to just cover until the rice is al dente and the liquid has been absorbed – about 10 mins. Season, then set aside.
Heat a large frying pan, then add the butter. Once melted, fry the mushrooms and onion until golden. Season, add the garlic, then cook for another couple of mins. Stir in the flour and cook for 1-2 mins, turning the vegetables over in it. Now take the pan off the heat and gradually add the milk, stirring to incorporate every addition before you add any more. Once done, put the pan back on the heat and bring to the boil, stirring all the time as the sauce thickens. Season well and let the sauce simmer so that the flour gets cooked. Stir in the cooked chicken and add the lemon juice, soured cream and dill.
Spread the rice in the bottom of 6 buttered individual pie dishes, then spoon the meat and sauce on top. Leave it to cool a little. Roll out the pastry a little and cut to fit your dishes, then pop on top
and press the pastry down. Trim off the excess. You can finish the pie crusts by making a long strip of pastry with the leftovers, twisting it like a rope. Moisten the edges of the pastry on the pies with
water, then press on the rope of pastry. Otherwise just crimp the edges.
Heat oven to 200C/180C fan/gas 6. Make 3 slits in the centre of the pies, then brush the tops with egg yolk. Bake for 10 mins, then turn the heat down to 180C/160C fan/gas 4 and cook for a further 20-25 mins, until the pastry is golden and cooked.