6 tablespoons butter1 cup button mushrooms finely chopped in a food processor or by hand3 leeks, washed and sliced1 red onion, finely sliced3 tablespoons chopped fresh tarragon½ cup white wine1 teaspoon kosher salt1 teaspoon ground black pepper2 puff pastry sheets, thawedFlour1 egg plus 2 teaspoons waterOne 3-pound salmon fillet, skin removedDijon Whipped Cream (recipe follows)Preheat the oven to 350 degrees. Melt the butter in a large skillet over medium heat. Add the mushrooms, leeks and red onion and saute until the mixture is soft, about 8 minutes.Add the wine, raise the heat to high and bring to a boil. Let the liquid cook off, add the salt, pepper and tarragon, then remove from the heat and set aside. Let cool before using.Combine egg and water to make egg wash. Unfold a pastry sheet and lay it on a lightly floured surface. Brush an 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.Makes 6 servings.