Fry the finely chopped onion in the preheated oil until Golden brown. Then add the chopped mushrooms and simmer until almost all the liquid has evaporated. All the liquid does not need to be evaporated-so the filling will be juicier. Salt and pepper. Boil buckwheat in salted water until tender. Leave the filling to cool.
Preparing the dough. Sift 2 types of flour, mix with salt and sugar, add cold oil and chop into small crumbs. Add milk and egg.
Knead the soft dough, roll into a ball, cover with a film and put in the refrigerator for 30-40 minutes.
Divide the cooled dough into 5 balls. On the floured surface, roll out circles with a diameter of about 15 cm.
Transfer the cakes on a baking sheet lined with oiled baking paper.
The blend in the center of the circle in the 1tablespoon of buckwheat.
Then on top of mushrooms with a small amount of liquid.
Put on top of 1 tablespoon of sour cream and grated cheese. Form the galette. Bake in preheated to 200 degrees. oven 25-30 min.