Flour, for dusting
1 sheet frozen puff pastry, from one 17-ounce package, thawed
2 small leeks (white and light green parts only), well rinsed, dried and thinly sliced (about 2 1/2 cups)
1/2 cup coarsely grated smoked Gouda cheese
1/4 cup freshly and finely grated Parmesan (use a microplane grater or the smallest holes on a box grater)
Kosher salt and freshly ground black pepper
2 tablespoons finely chopped chives