Preheat the oven to 450°F and set an oven rack in the middle position. Wash and scrub potatoes. Slice into 1 1/2″ slices then cut into 1 1/2″ chunks.
In a large pot, bring 3 quarts of water to a boil. Stir in 1 tablespoon of salt and the baking soda. Add the potatoes and boil for 10 minutes. Drain the potatoes thoroughly.
Directly on a rimmed baking sheet (avoid using foil as the potatoes may stick), toss the par-boiled potatoes with the potato starch, oil, ½ teaspoon of salt, half of the rosemary, and the pepper. Roast in the preheated oven for 35 minutes, stirring once midway through roasting. After roasting, turn off the oven but leave the potatoes inside for an additional 10 to 15 minutes.
Remove from the oven and sprinkle with the remaining rosemary. Taste and adjust the seasoning with salt, if necessary. Transfer the sweet potatoes to a platter and serve.
