Dilute sugar in warm water (0.5 cup), add yeast and leave the mixture for 10 minutes. Meanwhile, whip milk and kefir with 1 egg and 1 egg white (put 1 yolk aside to brush the pie), salt. Add half the flour and continue stirring. Pour the ripe yeast into this mass and stir again thoroughly. Gradually add flour and butter. The dough should be sufficiently elastic and easily come off the sides of the baking dish. Cover with a cling film or a damp clean tea towel. In 30-40 minutes, check how the dough has risen. You can immediately start forming the pie after the first rise of the dough, or you can let it rise 2 times more, since it will become more tender and "spongy". While the dough is actively rising, you can prepare the filling. Wash apples, peel, cut into cubes or slices and immediately sprinkle with lemon juice (or dip into ice water). Mix butter with sugar in a frying pan/saucepan, add apples and stew until softness for 5-10 minutes. Sprinkle with cinnamon, stir and allow to cool. Divide the ripe dough into 2 portions (2 pies), and also divide each portion into 2 unequal parts: a large ball for the pie and a small one for the pie cap. Leave to proof for 5 minutes. Roll out the size of the baking tray with a margin for folding. Carefully transfer the dough to the baking tray with a rolling pin. Spread the filling. Roll out a small piece of dough (the cap) and place on the filling. Join the edges of the dough and make a hole in the center of the cap. Brush with the beaten egg yolk. Leave to stand for 5 minutes and place in the oven preheated to 220 °C for 15 minutes. Once this time expires, reduce the heat to 190-200 °C and bake until done. If one side is baked faster, then turn the pie to another side. The pie is usually baked within 25-35 minutes. You should be guided by the smell and color of the product. Take the baked pie out of the oven, sprinkle with cold water or cover with a slightly damp towel and allow to stand (rest) for 15 minutes. The pie is ready to serve.