To prepare the filling. Cook hard-boiled 4 eggs. Finely chop onion, lightly fry it in butter, mix with chopped eggs and season with salt. Beat the eggs with a mixer. Add sour cream and mayonnaise, mix well in a mixer or whisk. Then add the sifted flour and mixed with baking soda and citric acid. Again mix well (the dough was as pancakes). Take a separable form, lubricated with oil. Slightly more than half of the dough pour in the form, then put the dough on the filling, flatten the filling and pour the second half of the dough. Bake in the oven at temperature 180 - 200 degrees (I baked 40 min.), a readiness test should come out clean or a toothpick. The filling can to make any. While I did just that. Very tasty. Reminiscent of cake, but for some reason the recipe called Pastida (casserole).