Line a baking sheet with foil and oil it. Set aside.
Cut the salmon into two equal portions and set aside.
Mix the butter with the lemon zest, basil, salt and pepper. Spread the herbed butter on one slice of salmon. Spread the mustard on the other piece of salmon. Sandwich the salmon together. Set aside.
Throw some flour on board and roll out pastry dough. Lay the salmon in the center of the dough. Trim the dough into a square leaving enough to cover the salmon.
Brush some egg yolk all around the fish. Fold the dough up lengthwise over the salmon. Tuck the sides in and trim excess. Score the dough with a knife and brush the remaining egg yolk over the whole salmon pocket.
Chill for 15 minutes. Preheat oven 400 degrees. Place the salmon on the baking sheet and bake for 30 minutes. until internal temp is 145 degrees. Rest it 5 minutes. Serve with hollandaise sauce