Preheat the oven to 350 degrees F.
Melt the butter in the olive oil in a large skillet over medium-high heat. Add the leek and tarragon and cook until the leek is wilted, about 5 minutes. Add the wine and bring to a boil. Let the liquid cook off, season with salt and pepper, remove from the heat, and set aside to cool.
Roll the pastry out on a floured surface until it's big enough to fold around the salmon. Spoon the cooled leek mixture down the center of the pastry and place the salmon on top of the leeks. Beat the egg lightly with 1 teaspoon water. Brush the edges of the pastry and fold the dough over the salmon; press the edges well to seal. Place the salmon, seam side down, on a baking sheet. Brush all over with the egg wash and bake until the pastry is puffed and golden brown, about 30 to 40 minutes.
Serve with the Pan Seared Beets and Beet Greens
Remove the beet greens, wash them well, and set aside.
Peel the beets and cut them into medium sized chunks. Put half of the oil into a large skillet and place it over high heat until hot, about 1 minute. Add the beets and cook for 5 minutes, shaking the skillet often. Reduce the heat to low, add the sugar, season with salt and pepper, and cover. Cook until tender, about 10 minutes; set aside and keep warm. Heat the remaining oil in the skillet over high heat for 1 minute. Add the beet greens all at once and season with salt and pepper. Cook just until the greens have wilted, about 2 minutes. Serve the beets and beet greens together.