In a deep bowl, mix the flour, water, butter and salt. Knead the dough that is left in the refrigerator for 2-3 hours. To make the dough greasy, mix the flour and butter. Place the mixture between two pieces of baking paper to get a square shape. Leave the dough prepared in this way in the refrigerator for 20 minutes to tighten the butter. Roll out the first dough, as well as the greasy dough into a long crust. Put them on top of each other and fold them 3 times. Leave the dough in the refrigerator for 1 hour. Repeat the procedure twice more. In a deep bowl, mix the egg, mascarpone and grated yellow cheese. Season with salt and ground black pepper to taste. Roll out the puff pastry to a thickness of 1 cm. Cut the crust into squares. In the middle, apply the filling, and and cot the ends with beaten egg. Bake in a preheated oven at 200°C for about 20 minutes. Before serving, add the young spinach on top of each piece.