Prepare the necessary ingredients. Rinse the champignons well under running water and remove any dirt from the caps. Wash the cherry tomatoes and eggs well. Place the butter in the freezer for 10 minutes. Preheat the oven to 180 degrees.
Remove the butter from the freezer and grate it on a coarse grater. Also cut the vegetables and cheese for the filling: cut the champignons into slices, finely chop the onion and cut the cheese into squares.
Sift the flour and baking powder into a deep bowl, add the grated butter and, using your fingers, mix the butter with the flour so that butter-flour crumbs form.
Add salt and sour cream, knead the dough.
Form the dough into a ball, wrap it in cling film and place it in the refrigerator.
Heat a frying pan with a small amount of vegetable oil or butter, add the onion. Fry, stirring, for several minutes.
Add chopped champignons to the onion, add salt and pepper to taste, stir and continue frying, stirring occasionally, for another 10-15 minutes.
Pour milk into a deep bowl, add eggs and salt and pepper, beat until smooth.
Remove the dough from the refrigerator and roll out into a circle about 1-1.5 cm thick.
Place the dough in a removable pan, greased with oil. Form the sides and pierce the bottom with a fork in several places. Place the base in an oven preheated to 180 degrees for 10 minutes.
Place mushrooms, cherry tomatoes and Feta cubes, fried with onions, into the slightly baked sandy base of the pie. Pour the filling with the milk-egg mixture and return to the oven preheated to 180 degrees for 35 minutes until cooked.
The pie itself is very bright, beautiful and aromatic, so it makes an impression without additional decorations. Serve slightly or completely cool. Bon appetit!