Preheat your oven to 350F and line a baking sheet with parchment paper. Reserve.
Take the salmon filet and place on the prepared baking sheet. Lightly salt & pepper the flesh side. Bake for 12-15 minutes, or until cooked through. Place aside.
While the fish is cooking, heat a medium saucepan over medium high heat. Add 1/2 tablespoon of olive oil. Once heated, add your washed and chopped mushrooms to the saucepan. Cook until slightly browned and reduced in size, about 7 minutes.
In a large bowl, combine the already cooked rice, mushrooms, and 3 tablespoons of prepared pesto. Flake the cooked salmon into the mix. Add 1 tablespoon of chopped dill and mix well. Salt & pepper to taste. Reserve.
Prepare a baking sheet lined with parchment paper.
Raise the oven temperature to 400F.
Take a sheet of puff pastry and gently roll to fit the size of a standard baking sheet. Make sure the dough stays chilled. Transfer to the lined baking pan.
Spoon in the salmon/mushroom/rice mixture down the center length of the rolled out dough. Gently fold in the sides and pinch the bottoms together to seal the filling in. If you like, carefully flip the log over, so the seam is on the pan. Fold the bottoms once more.
Mix 1 egg with 1 tablespoon of water in a small bowl and using a brush, coat the pastry with the egg wash.
Bake 15-17 minutes, or until puffed and golden.
Enjoy hot or room temperature.