2 1/2 pounds (1.1kg) sweet potatoes, peeled and sliced crosswise into 1/2-inch-thick rounds
1 cup light brown sugar (about 7 ounces, lightly packed; 200g)
1/4 cup unsalted butter (2 ounces; 60g)
1/2 cup (120ml) water
1 tablespoon (12g) Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
1 tablespoon (15ml) apple cider vinegar
1/8 teaspoon ground cinnamon
1/8 teaspoon ground allspice
1/8 teaspoon ground ginger
Making a proper syrup first keeps the syrup from separating in the oven.
In a 9- by 13-inch baking dish, arrange sweet potato rounds in an even layer, overlapping where necessary to fit them all. Set aside.
In a small saucepan, combine brown sugar, butter, water, and salt and bring to a boil, stirring often, until syrup registers 220°F (104°C) on an instant-read thermometer. Stir in cider vinegar, cinnamon, allspice, and ginger, then pour syrup all over potatoes, spooning it over them as necessary to wet them completely.
Cover baking dish with aluminum foil and place in a cold oven. Turn on oven to 250°F (120°C) and allow it to come up to temperature. Cook, stopping to flip and baste potatoes with syrup every 20 minutes, until tender throughout, about 1 hour 15 minutes. (Exact cooking time will depend on your oven, baking dish, and the size of your potatoes, so check frequently.)
Remove foil and continue to cook, flipping and basting potatoes occasionally, until they are very soft and glazed in a rich brown syrup, about 30 minutes longer. Let cool slightly, then serve