Clean and wash the chard, strip the green from the stalks and chop. Cut the stems into strips.
Sauté the onions and stems in the fat and cook for 10 minutes, then add the leaves and cook for another 5 minutes. Thaw the puff pastry sheets side by side.
Mix the cheese with the chard. Season with salt, pepper and nutmeg.
Roll out each sheet of puff pastry to approx. 20 x 8 cm and cut it into thirds. Place 1 teaspoon of the chard and cheese mixture on each, brush the edge with water, fold over the other side and press firmly. Brush with egg yolk.
Place the bags on a baking sheet rinsed with water and bake in a preheated oven at 225 ° (fan oven 200 °) for about 15-20 minutes.