Preheat the grill to medium-high.
Place the potatoes in the center of a double thickness of heavy aluminum foil. Toss with 1 to 1 1/2 tablespoons of the oil and season lightly with salt and pepper. Wrap up in the foil, turning up the edges to seal. Place the potatoes on the grill and cook until tender, 25 to 30 minutes. Remove and cut into wedges.
Meanwhile, brush the fennel and onion slices on both sides with the remaining olive oil and lightly season with salt and pepper. Place on the grill and cook until marked and tender, and starting to turn golden brown, 4 to 5 minutes per side. Remove from the grill and place on a cutting board. Roughly chop.
Combine the potatoes, fennel and onions in a large bowl and while warm, toss with the Herbed Buttermilk Dressing. Adjust the seasoning, to taste, and serve immediately.
Yield: 6 servings