For the dough, dissolve the yeast and sugar in lukewarm milk in a bowl and let stand for about 2 - 5 minutes. Then knead with the remaining ingredients to form a smooth dough. Add a little more water if necessary. Cover and let the dough rise in a warm place for at least 60-120 minutes.
For the filling, fry the carrot, onion, tomato and parsley. Add the feta cheese to the tomato mixture. Put the spinach in a saucepan, bring to the boil and season.
Knead the dough a little more on a smooth work surface without flour, divide into 10 portions and cover with a kitchen towel. Roll out the pieces into thin, oblong-oval flat cakes. Spread a good portion of the filling in the middle of the thin dough. First a small portion of spinach, then the tomatoes with feta, on top a little Gouda. Then fold the edges inwards approx. 1.5 cm and twist the two ends slightly.
Place the pastries on a baking tray lined with baking paper. Spread a thin layer of beaten egg yolk on the edges of the dough and sprinkle with a little black cumin and / or sesame seeds.
Bake in a preheated oven at 180 ° C top / bottom heat for approx. 20 - 25 minutes until they are golden brown.
Serve the Börek fresh from the oven with tzatziki and cucumber.