Preheat the oven to 350 degrees F.
Melt the butter in a large skillet over medium heat. Add the leeks, red onion and tarragon and saute until the onion mix is soft, about 10 minutes. Add the wine, raise the heat to high and bring to a boil. Let the liquid cook off, add the salt and pepper, and remove from the heat and set aside. Let cool before using.
Unfold 1 pastry sheet and lay it on a lightly floured surface. Brush 1 edge with the egg wash and overlap the other sheet by 2 inches. Roll the pastry out until it's big enough to fold around the salmon. Spoon the cooled leek mixture down the center of the pastry, place the salmon on top of the leeks and fold the pastry over the salmon. Egg wash the edges to seal and press well. Place the salmon parcel seam side down on a sheet pan. Brush with egg wash and place in the oven for 30 to 40 minutes, until the pastry is puffed and golden brown. Remove from the oven and let sit for 15 minutes before slicing. Slice and serve with Dijon Whipped Cream.
Pour the cream into a chilled stainless steel bowl and whisk until soft peaks begin to form. Add the mustard, salt and pepper and continue to whisk until stiff peaks form. Refrigerate until ready to use.
Recipe courtesy of Dan Smith and Steve McDonagh