On the grill, heat a large cast iron skillet until almost smoking add olive oil and onions. Stir onions until cooked. Sear potatoes in skillet with onions for 1 minute, cut side down, or until brown and crisp. Remove from heat and transfer potato mixture to a mixing bowl. Set the skillet aside. In a blender, combine all vinaigrette ingredients except olive oil, and blend until smooth. While the motor is still running, slowly add olive oil in a thin stream until vinaigrette is emulsified. Season to taste with salt and pepper. In the mixing bowl combine the potato and onions. Toss with Lime-Garlic Vinaigrette. Season with salt and pepper. Serve in cast iron skillet.