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 two bowls. Dividing evenly between the bowls, fold in the sweet potatoes, Gruyere, and thyme; season each with 1 1/2 teaspoons salt and 1/2 teaspoons black 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 pie rust (about 1/2 cup per piecrust), leaving a 1/2-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 egg.

If for 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-lend 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 parchment-lined baking sheet at 400*F until golden brow, 30 to 35 minutes.