Mix kefir, water, salt and soda (unslaked) and add flour so much as to make soft dough. Knead it well and set aside for a while, having covered with a towel. Then boil potatoes and mash them while hot. It is very important for the puree to be smooth, since the lumps will tear the dough. Grate cheese and add to the hot mashed potatoes. Stir well. The filling and dough should be nearly equal. Shape balls from dough 4-5 cm in diameter. Roll out a ball as thin as possible and put stuffing on it. Next, pick up the edges of the dough and pinch them over the filling. Make a flat cake from the resulting ball with your fingers. Then gently roll it out with a rolling pin to the thickness of 3-5 mm on a floured table. Put the flat cake on a heated frying pan and bake on a medium heat (do not oil the frying pan). Turn it over in 1-2 minutes. If the flat cake starts to bloat out, poke it with a knife. Immediately butter the ready khychin and put in a lockable container. Before serving, you can cut a pile of khychiny into four parts. For sauce: mix sour cream, ayran (kefir) with crushed garlic and finely chopped dill. Add salt and pepper to taste. Serve along with khychiny.