3 pounds (1.4kg) sweet potatoes, peeled and cut into 1/2-inch chunks
4 tablespoons (60ml) vegetable or canola oil, divided
Kosher salt and freshly ground black pepper
2 tablespoons (30ml) miso (we used white miso during testing, but you can use any type)
1 tablespoon (15ml) mirin
1/2 tablespoon (8g) sugar
2 tablespoons (30ml) water
2 scallions, white and light-green parts only, thinly sliced
Shichimi togarashi, for garnish (optional; see note above)
Place sweet potatoes in a large saucepan and cover with cold water. Heat water to 160°F (70°C) as registered on an instant-read thermometer. Cover and set aside for 1 hour.
Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 400°F (200°C). Drain sweet potatoes and transfer to a large bowl. Toss with 3 tablespoons (45ml) oil and season to taste with salt and pepper. Spread sweet potatoes on 2 rimmed baking sheets and roast until bottom sides are browned, about 30 minutes. Carefully flip potatoes with a thin spatula and roast until second side is browned and potatoes are tender, about 20 minutes longer. Keep potatoes warm.
When ready to serve, in a large mixing bowl, whisk together miso, mirin, sugar, water, and remaining 1 tablespoon (15ml) oil until smooth. Add potatoes and gently toss to coat. Fold in scallions and transfer to a serving bowl. Sprinkle with shichimi togarashi, if using, and serve.