Combine flour, salt, and butter in a bowl and knead with your hands. Add 3 tablespoons of sour cream and continue kneading until the dough stops sticking to the sides of the bowl. Form the dough into a ball and refrigerate for 3 hours. Roll out the chilled dough on a lightly floured board, place it in the pan, and seal the edges.

Mix the thinly sliced onion with 1 teaspoon of salt, sugar, and black pepper, and sauté in butter until the onion is soft but not browned.

Beat the eggs, sour cream, remaining salt, red pepper, and nutmeg together, then mix with the sautéed onions.

Pour this mixture onto the dough, and place the pan in a well-heated oven at 180°C for 40 minutes or until the dough is browned.

Serve hot.