Preheat your oven to 450°F (230°C) and set the oven rack in the middle position.
In a large pot, bring 3 quarts (3 L) of water to a boil. Stir in 1 tablespoon of salt and the baking soda. Add the sweet potatoes and boil for 10 minutes. Drain the potatoes thoroughly.
Directly on a rimmed baking sheet, toss the par-boiled sweet potatoes with potato starch, olive oil, ½ teaspoon of salt, half of the fresh rosemary, and the black 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 seasoning with salt if necessary. Transfer the sweet potatoes to a platter and serve.