Easy Apple Turnovers

Name: Easy Apple Turnovers
Ingredients: 1 (about 16-ounce) package frozen puff pastry; 1 large egg; 1 1/2 pounds Granny Smith or Pink Lady apples (3 to 5 apples); 3 tablespoons water, divided; 2 tablespoons unsalted butter; 1/4 cup packed dark brown sugar; 2 teaspoons apple cider vinegar or lemon juice; 3/4 teaspoon ground cinnamon; 1/4 teaspoon ground nutmeg; 1/4 teaspoon kosher salt; 1 tablespoon all-purpose flour, plus more for dusting; 1 teaspoon vanilla extract; Turbinado sugar or sanding sugar, for sprinkling (optional)