Leave your frozen puff pastry on the counter to thaw until cold, yet still rollable. Keep it cold! Just not frozen!!!
Dry off your salmon fillet, ensure no bones remain, and rub with a dijon mustard, coat with olive oil, and rub in. Set aside.
Blend a brick of softened cream cheese, a good handful of fresh chopped basil & fresh chopped dill in a bowl. Set aside.
Sautee 1/2 a red onionin a knob of butter, and as the onion becomes translucent, add a few cloves of crushed fresh garlic, and 2 heads of spinach. Sautee on medium until everything is moderately cooked and spinach is fully wilted. Set aside.
Roll out the puff pastry, add on a layer of your spinach, garlic & red onion mix. Make it just a tad smaller than the footprint of your salmon fillet.
Put your mustard rubbed salmon fillet on top of the mixture, then add a healthy layer of the cream cheese mixture.
Egg wash the exposed edges of your puff pastry, fold it over your beautiful little concoction, punch the seams closed, and brush the entire thing with egg wash.
Feel free to use some additional puff pastry to make a lattice cover, or something fancy as you see fit. Egg wash again.
Put this thing on a greased cookie sheet, and into an oven pre-heated to 425 for about 25 mins or until the outer crush is a nice brown.
Remove from oven, let sit for a few minutes, and cut with a bread knife.