Place puff pastry slices next to each other and defrost at room temperature. In the meantime peel onions and cut them into thin slices. Drain tuna and capers well. Pick the tuna apart with a fork, remove the stalks of the capers. Stir the crème fraîche until smooth and season with salt and pepper
Place 2 puff pastry slices on top of each other and roll out square (approx. 18 x 18 cm) on a floured work surface. Cut each into quarters. Fold the dough corners of the 8 squares about halfway inwards and press firmly. Spread the crème fraîche on the dough. Cover with onions, capers and tuna
Mix the egg yolk and cream. Brush the edges of the dough with egg cream. Sprinkle Copenhagener with cheese. Place on a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 20 minutes until golden brown
Clean, wash and cut the spring onions into rings. Take Copenhagen out of the oven, sprinkle with spring onions and serve immediately