Start by making the dough. Mix the flour, salt and butter. Work with your finger until it becomes a crumbly mixture. Add the yoghurt and form a soft but firm round. Add more flour if necessary. Cover the dough with a plastic wrap and let it chill in the refrigerator for at least half an hour, or overnight if possible.
Peel the apples and core them. Cut them in cubes (approx. 2x2) and set aside. Melt the butter in a pan and add the sage leaves. Cook until the butter is browned. Add the apples and stir continuously for 6-7 minutes. Add the spices and stir to cover the apples evenly. Lastly, add the sugar and let it caramelize for a few minutes, then remove from the heat.
Preheat the oven to 180°C. Roll the whole dough to 35 cm round or cut it into 4 pieces and roll out into 12 cm rounds. Place the dough(s) on a baking sheet covered with paper. Spread the apple filling in the middle of the dough(s) leaving a 4 cm frame (2 cm for the small ones). Fold the dough over the filling and glaze with the egg yolk. Sprinkle some sugar over the apples and the dough. Bake until the dough is golden-brown, for about 25-30 minutes. Let it cool for 10 minutes, then serve with vanilla ice cream.