Preheat oven to 400°.
Line a sheet pan with parchment paper. Set aside.
In a medium saute pan, melt butter over medium-low heat. Add leeks and sauté until tender but not browned, about 4 minutes. Stir in crème fraîche and simmer 1 minute. Set aside
In a medium bowl, mix ham and Gruyère. Add leek mixture, spinach, basil, Parmesan, salt, and black pepper. Mix and set aside.
In a small bowl, whisk together egg and milk
Sprinkle a cutting board with flour, roll out 1 sheet of the puff pastry to an 11" x 11" square, and cut into 4 equal squares. Trim the edges for clean sides. (Tip: Use a pizza cutter to cut the pastry dough.)
Brush edges of each square with egg wash. Place 1/3 cup of the ham and cheese filling in the center of each square. Fold each square diagonally to form a triangle, lining up edges of pastry.
Press edges with tines of a fork to seal.
Place empanadas on prepared sheet pans and brush each with egg wash and sprinkle with fleur de sel and pepper.
Bake until puffed and golden brown, 20 to 25 minutes. Allow to cool 5 minutes before serving.