1. Preheat oven to 400ºF.
2. Get yourself a large, flat sheet pan and dust it with flour. Dust a clean surface and a rolling pin with flour. Lay the puff pastry sheets one on top of the other. Roll out your puff pastry, dusting as you go, until it’s the same size as the sheet pan (about 12in x 6in).
3. Place the pastry on your floured pan. Drizzle the salmon fillet with olive oil and season with a pinch of salt and pepper.
4. Transfer to the pastry and place red onions on top. Slice the tomatoes and place them over the onions.
5. Put half of the kalamata olives on top of the tomatoes and scatter the rest around the edges of the salmon fillet.
6. Tear the mozzarella into pieces and scatter these on top. Pick the basil leaves and place them over the mozzarella. Sprinkle with salt and pepper and drizzle with olive oil.
7. Gather up the sides of the pastry and pinch and push them together.
8. Crack egg into a cup and beat it with a fork, then use a pastry brush to paint this egg wash all around the pastry edges.

To cook and serve your salmon en croute:

1. Place the sheet pan at the very bottom of the preheated oven, with an empty sheet pan or cookie sheet on the shelf above to protect the top of it from getting too much heat.
2. Cook for 35 minutes, then remove from the oven and serve immediately.