Rinse black cabbage with cold water, drain and cut into fine strips. Peel and finely chop onion and garlic clove. Break the feta into small pieces.
Heat olive oil in a pan and fry the onions briefly. Add black cabbage and caraway seeds and fry briefly. Now deglaze with a splash of vinegar. Reduce and season with nutmeg, salt and pepper. Then add the feta pieces.
Prepare the spelt puff pastry according to package instructions. Cut into 6 x 6 cm squares.
Place a mound of filling in the center of each, brush the edges with egg yolk and fold the puff pastry pieces into triangles. Press down with a fork.
Place them a little apart on a baking tray lined with baking paper and brush with beaten egg.
Bake the dumplings in the preheated oven at 190 °C for approx. 12-14 minutes.