For the pastry, pile the flour on a work surface, mix with the salt and make a well in the center. Cut the cold butter into small pieces and distribute around the well. Add the egg to the center and bring all the ingredients together with a knife to form crumbs.
Knead into a dough with your hands and form into a ball. Wrap in plastic wrap and refrigerate for 30 minutes.
For the filling, rinse, trim and chop the scallions.
Peel, chop and sauté the shallots in some butter. Season with salt, pepper and caraway. Add the scallions.
Place the dough between 2 layers of parchment paper and roll out. Lay in the greased tart pan and place in a preheated oven at 175ºC (approximately 350ºF).
Bake for about 5 minutes.
For the filling, combine the cream, sour cream and eggs and season with salt and pepper. Distribute the onion mixture into the prebaked tartlet and pour over the cream mixture.
Bake for another 12-15 minutes
Serve hot or cold.