Peel the sweet potatoes, half, and slice them into about 1/4 half circles. Place in a saucepan with 4 cups of water.
Stir in the white and brown sugar, cinnamon, ginger, orange zest, salt and butter. Bring to a boil, reduce heat and simmer uncovered for about 10-12 minutes. Sweet potatoes should be tender, but still firm.
Grease a large baking dish or a large cast iron skillet. Transfer the cooked potatoes to the baking dish (it's better to use a slotted spoon for that, because we want to leave liquid in the saucepan). Turn the heat on the saucepan to high and boil, stirring occasionally, until the liquid is reduced by half and slightly thickened. It should take about 15-20 minutes and you should get about 1 cup of the liquid. Pour it over the sweet potatoes in the baking dish.