Heat the butter in a frying pan, add the asparagus and fry for 2–3 minutes until just softened, then season with salt and pepper. Drizzle the salmon with lemon juice and season.
Roll the pastry out thinly on a lightly floured surface and trim to a 35 cm (14 inch) square, then trim a 3.5 cm (1½ inch) strip off one of the sides. Put the salmon on top so that the long side of the salmon is parallel with the narrower side of the pastry rectangle.
Dot the cheese on top of the salmon, then sprinkle with the lemon rind, herbs, peppercorns if using, and tomatoes. Arrange the asparagus on top, alternating their direction.
Brush the pastry around the salmon with beaten egg, then fold the narrower sides up and over the salmon, pressing to seal. Cut the excess from the top ends of pastry, then fold and press to wrap the salmon like a parcel.
Brush the parcel with beaten egg and transfer to a baking sheet. Decorate (see