Peel and finely chop the onion. Heat the fat in a saucepan and braise the onions and sauerkraut. Add apple juice and braise for another 10 minutes. Season to taste with salt and pepper, drain if necessary and let cool off.
Place puff pastry sheets next to each other and allow to thaw. Drain and separate the sausage chain. Mix crème fraîche, mustard and a little salt. Place the puff pastry sheets on top of each other and roll out into a rectangle (28x28 cm).
Cut the dough into squares of about 7x7 cm. Put some sauerkraut and mustard crème fraîche on each square and place a cocktail sausage diagonally on top. Fold over 2 dough corners and pin them with a wooden skewer (toothpick).
Place the particles on a baking tray lined with baking paper and bake in a preheated oven (electric: 200 °C/ gas: level 3) for 15-20 minutes. Mix egg yolk and cream and brush with it about 5 minutes before the end of the baking time.
Clean the salad, wash and drain well. Arrange salad and sausages on a plate. Garnish with parsley and serve with cucumber slices.