Clean, rinse and drain the black cabbage. Pull the cabbage leaves from the thick, medium stems and cut into fine strips. Finely chop the onion and garlic. Heat oil in a pan and sauté onions and garlic. Add the black cabbage and cook over medium heat for 4-5 minutes until the cabbage has wilted. Season with caraway, salt, pepper and nutmeg. Crumble the feta and mix in.
Unroll the puff pastry, cut it into 15 squares (8 x 8 cm) and chill for 10 minutes (it works best when chilled). Place a tablespoon of the black cabbage filling in the middle of each square. Whisk the egg yolk with 2 tablespoons of cold water and brush the edges of the puff pastry squares with a little egg yolk. Fold the squares into triangles and press the edge with a fork. Place the puff pastry corners a little apart on a baking tray lined with baking paper and brush with the remaining beaten egg yolk. Bake in a preheated oven at 200 degrees (fan oven 180 degrees) on the second rack from the bottom for 12-15 minutes until golden.
Rinse the chives and parsley and shake dry. Cut the chives into small rolls. Pull the parsley from the stems and chop finely except for a few leaves. Mix the sour cream with the herbs, salt and pepper. Remove the puff pastry corners from the oven, serve with the herb dip and serve sprinkled with the remaining parsley.