Preheat the oven to 200C/400F.
Chop the kale (thick ribs removed) very finely and mix with softened mascarpone or cream cheese, add lemon juice, salt and pepper. The mixture should resemble a thick paste.
Roll out the puff pastry on a lightly floured surface into a rectangle appox 1/8" thick. Place the salmon pieces in the middle of the pastry rectangle, salt, pepper and spread the kale mixture directly on them.
Brush the edges with beaten egg, fold the sides and pinch them together securely to resemble an envelope. I used a fork to make sure all juices stay in.
Flip the salmon filled pastry over and place it on parchment paper lined baking sheet and brush it with the egg wash all over.
Optional: Make decorations out leftover pastry and place on top, brush with egg wash.
Make 3-4 little slits for ventilation.
Cook in the preheated oven for 30 minutes until pastry is puffed up and golden.
Let it sit for 5 minutes before serving.