Brush 1 thickly sliced onion and 1 bunch scallions with vegetable oil and season with salt. Grill over medium-high heat, covered, until tender, 2 minutes for the scallions and 10 minutes for the onion; chop. Boil 1 1/2 pounds halved small red-skinned potatoes until tender, 10 to 15 minutes; drain and let cool. Mix 1/3 cup sour cream and 3 tablespoons each buttermilk and horseradish in a bowl. Stir in the potatoes, onion and scallions. Season with salt and pepper.