"A Barefoot Contessa Cookbook" recipe.
Preheat the oven to 400 degrees. Line 2 sheet pans with parchment paper.
Melt the butter over medium-low heat in a medium (10-inch) sauté pan. Ass the leeks and sauté for 4 minutes, until tender but not browned. Stir in the crème fraîche and simmer for a minute.
Combine the ham and Gruyère in a medium bowl. Add the leek mixture, then the spinach, basil, Parmesan, 1¼ teaspoons salt, and ½ teaspoon pepper. Mix and set aside.
Sprinkle a cutting board with flour, roll out one sheet of pastry to an 11 x 11-inch square, and cut into 4 equal squares. Brush the edge of each square with the egg wash and place ⅓ cup of the filling in the middle. Fold the square diagonally to make a triangle, lining up the edges of the pastry. Place the triangles on the prepared sheet pans and press the edges with the tines if a fork to seal. Brush the triangles with the egg wash and sprinkle with fleur de sel and pepper. Repeat with the remaining pastry and filling. Bake for 20 to 25 minutes, until puffed and golden brown. Allow to cool for 5 minutes and serve hot.