How to Make It
Heat oven to 400° F. Heat the oil in a large saucepan over medium-high heat. Add the onion and cook, stirring often, until beginning to soften, 4 to 5 minutes. Add the chard and cook, stirring, until wilted, 2 to 3 minutes. Transfer to a large bowl and fold in the sweet potato, Gruyère, and thyme; season with ¾ teaspoon salt and ¼ teaspoon pepper. Spread the vegetable mixture on a rimmed baking sheet and let cool for 10 minutes.
Dividing evenly, spoon the sweet potato mixture (about ½ cup) on one side of each half-circle of piecrust, leaving a ½-inch border. Dot the borders with water, fold the dough over to form a quarter-circle, and press firmly to seal; crimp, if desired. Cut 3 slits in each turnover.
Transfer the pies to a parchment-lined baking sheet, brush with the egg, and bake until golden brown, 20 to 25 minutes. Serve with the salad, if desired.