Place the puff pastry slices next to each other and let them thaw at room temperature for about 20 minutes. Wash the salmon fillet, dab dry. Remove any bones with tweezers. Sprinkle salmon with lemon juice.
Place the puff pastry slices on top of each other. Roll out to a rectangle on a floured baking tray so that it is at least twice as large as the salmon fillet. Spread one half of the pastry with breadcrumbs. Dab salmon dry, salt and place on the breadcrumbs.
Separate the egg. Whisk egg whites. Spread the edges of the dough. Place the other half of the dough over the salmon, pressing the edges well together. Mix egg yolk and 2 tablespoons of whipped cream. Brush the surface of the dough with it.
Bake salmon in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-30 minutes. In the meantime, place the remaining dough on top of each other and roll out again. To garnish, cut out 6 fish, brush with whisked egg yolk.
Bake the last five minutes with it. Heat the butter in a pot. Sauté 2 tablespoons of flour in it until golden brown. Stir in 300 ml water, remaining cream and stock and simmer for about 5 minutes. Season to taste with salt, pepper and possibly lemon juice.
Wash the dill, shake dry and cut finely. Stir into the sauce. Cut salmon into pieces, arrange and garnish with the fish.