Caramelized Onion Twice-Baked Potatoes

Heat oven to 375 degrees F. Rub the potatoes with 1 tablespoon oil, sprinkle with 1/2 teaspoon salt and transfer to a large baking sheet. Bake until tender in the center, about 1 hour 15 minutes. While the potatoes are baking, heat the remaining oil in a large skillet over medium heat. Add the red onion and cook, stirring occasionally, until tender and lightly golden brown, 15 minutes. Once the potatoes are cool enough to handle, cut them in half crosswise. Scoop out the insides, leaving a 1/8-inch border, and transfer to a large bowl. Mash with the sour cream and 1/2 teaspoon each salt and pepper. Fold in the scallions and onions. If necessary, slice a thin piece off the rounded sides of each potato so they can stand upright. Transfer the potato skins back to the baking sheet. Divide the filling among the potato halves (about 1/2 cup each) and bake until heated through, 15 to 20 minutes. Sprinkle with additional scallions and pepper, if desired.