For the dough, Place the flour, sugar, and salt in a bowl. Make a well in the center. Cut the cold butter into small pieces and place in the well, add the egg in the center, and about 2-3 tablespoons of water. Cut the ingredients together until coarse crumbs form using a pastry cutter or 2 knives. Form the dough into a disk, cover and refrigerate about 30 minutes.
Preheat the oven to 180°C (approximately 350°F). Rinse, peel, halve and core the apples and pears. Score the rounded side of each half fruit by slicing lengthwise while still leaving the top and bottom intact. Immediately sprinkle with lemon juice.
For the filling, separate the eggs. Beat the yolks and sugar until frothy and light in color. Mix together the quark, the vanilla bean seeds, and the cornstarch. Beat the egg whites with a pinch of salt until stiff peaks form. Combine the yolk mixture and the quark mixture. Fold the egg whites into the into the new quark mixture.
Grease and flour the tart pan. Roll out the pastry on a lightly floured surface until slightly larger than the mold and place it in the mold. Trim the edges. Prick the dough several times with a fork. Evenly spread the quark mixture on the dough. Press the pear and apple halves with the rounded and scored side up into the quark mixture in a decorative shape. Sprinkle with cinnamon and sugar and bake until golden brown, about 30-35 minutes. Serve hot with coffee if desired.