- 3 large onions
- 3pounds sweet potatoes
- 1/4cup olive oil
- 4 bunches Swiss chard, stems discarded and leaves cut into 1/2-inch strips (about 24 cups)
- 1pound Gruyère, grated (4 cups)
- 2tablespoons fresh thyme leaves
- kosher salt and black pepper
- 8 refrigerated rolled piecrusts, each cut in half (to form 16 half-circles)
- 1 large egg, beaten
- In a food processor fitted with the large grating disk, grate the onions; transfer to a bowl. Grate the sweet potatoes and transfer to a second bowl.
- Divide the oil between 2 large pots and heat over medium-high heat. Divide the onions between the pots and cook, stirring often, until beginning to soften, 8 to 10 minutes. Divide the chard between the pots and cook, stirring, until wilted, 5 to 7 minutes.
- Divide the vegetable mixture between 2 bowls. Dividing evenly between the bowls, fold in the sweet potatoes, Gruyère, and thyme; season each with 1 ½ teaspoons salt and ½ teaspoon pepper. Spread the sweet potato mixture from both bowls onto a rimmed baking sheet and let cool for 10 minutes.
- Dividing evenly, spoon the mixture onto one side of each half-circle of piecrust (about ½ cup per 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 and brush with the egg.
- To eat tonight: Bake the turnovers on a parchment-lined baking sheet at 400° F until golden brown, 20 to 25 minutes.
- To freeze and cook later: Freeze the uncooked turnovers on a parchment-lined baking sheet just until firm, 15 to 20 minutes. Transfer to freezer-safe resealable bags and freeze for up to 3 months. To cook, bake the turnovers from frozen on a parchment-lined baking sheet at 400° F until golden brown, 30 to 35 minutes.