Miso Scallion Roasted Sweet Potatoes

Name: Miso Scallion Roasted Sweet Potatoes
Ingredients: 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 notes)