Preheat oven to 400 degrees, with a rack in center. Place potato on rack; bake until tender when pierced with a knife, about 1 hour. When cool enough to handle, remove potato skin and mash until smooth. Combine 1 cup mashed potato with feta and chives. Season with salt and pepper. Whisk 1 egg; stir into mixture.Advertisement
On a lightly floured surface, roll pastry into a rectangle; cut into six 4-inch squares. Place 1 heaping tablespoon of filling in corner of each square, leaving a 1/2-inch border. Whisk remaining egg with 1 teaspoon water. Brush borders with egg wash; fold dough over filling and press edges to seal, forming triangles. Crimp edges with a floured fork. With the tip of a sharp knife, cut vents in each pastry. Arrange turnovers on a parchment-lined baking sheet and freeze until firm, 20 minutes (or, wrapped in plastic, up to 2 days).
Brush turnovers with egg wash; sprinkle with seeds. Bake, rotating sheet halfway through, until deep golden brown and puffed, 25 to 27 minutes. Transfer using parchment to a wire rack; let cool completely.
This recipe is also a great way to use up leftover cooked potato; start with one cup of mashed to make the filling.