Using an electric mixer, whip the cream cheese and sour cream in a small bowl until fluffy. Add the granulated sugar and whip until well combined, making sure there are no lumps. Reserve for later use.
Beat the egg and 1 tablespoon water in a small bowl with a fork.
Unfold 1 pastry sheet on a lightly floured flat surface. Roll the pastry into a 14-by-12-inch rectangle. Spoon half of the cream cheese mixture lengthwise down the center of the pastry. Cut slits 1 inch apart from the 2 sides of the pastry rectangle within 1/2 inch of the cream cheese mixture. Arrange half of the strawberries down the center of the pastry on top of the cream cheese mixture. Starting at one end, fold the pastry strips over the strawberries on a diagonal, alternating sides, to cover the strawberries. Repeat the entire process with the remaining pastry sheet, cream cheese filling and strawberries. Brush the pastries very lightly with the egg wash (otherwise they won't rise). Sprinkle with the demerara sugar and top with pistachios.
Bake until golden brown, 30 minutes. Let cool on a rack for about 20 minutes before serving.