Preheat your oven to 225°C or about 425°F. Begin by cleaning your salmon of all the bones and scales. If there is excess oil seeping from the skin, pat that down with a paper towel.
Take the puff pastry and cut it into 2 sheets. On a baking tray lay down a piece of parchment paper, then a sheet of puff pastry, then the slab of salmon.
In a ramekin or small dish, mash together the butter with the chopped herbs and garlic powder. I just use a fork until everything is combined.
Spoon the herb butter mixture over the salmon pressing it down to cover most of the meat.
Take the other sheet of puff pastry and lie it over the fish completely covering it. Using your fingers, or a fork, press the two layers together to completely seal it. Cross hatch some cuts on top being careful not to completely cut through the top sheet. As you can see by this picture, I actually had 4 small sheets of puff pastry that I fused together to make 2 larger sized ones. This isn't a problem as long as they can be fused well together.
In a small ramekin or bowl, beat the one egg. Take a pastry brush and give the top layer an egg wash, making sure to get every part of the dough. Sprinkle some coarse salt over the top.
Place in the oven for 22-25 minutes, or until the top is a golden brown. Remove and let rest for 5 minutes before cutting it into slices with a bread knife.