How to Make It
Preheat oven to 425°F. Pierce potatoes several times with a fork. Arrange in a single layer in a rimmed baking sheet, and bake in preheated oven until just tender, 25 to 30 minutes. Transfer to a wire rack; cool completely, about 30 minutes.
Meanwhile, combine pineapple juice, mirin, sake, soy sauce, and brown sugar in a small saucepan over medium, and simmer, stirring often, until thickened, 10 to 15 minutes. Remove from heat; set aside.
Peel sweet potatoes, and cut crosswise into 1-inch pieces, discarding ends.
Preheat grill to medium (350°F to 400°F). Skewer sweet potato pieces. Brush with oil, and sprinkle with salt and pepper; grill, uncovered, 3 minutes. Brush with some of the pineapple glaze, and continue grilling, turning every 2 to 3 minutes and brushing with glaze after each turn, until charred on both sides. Transfer to a serving platter, and sprinkle with scallions; drizzle evenly with 3/4 cup of the pineapple glaze. Serve immediately with remaining glaze.