Preheat the oven to 450 degrees F. Line a large shallow roasting pan with nonstick aluminum foil.
In a large bowl, combine the potatoes, olive oil, garlic, 1 teaspoon salt, and 1/2 teaspoon pepper, tossing gently to coat. Arrange the potatoes, in a single layer, on the prepared pan. Bake until potatoes are tender and lightly browned, stirring occasionally, for 35 minutes. Remove from the oven and cool.
In a large skillet, melt the butter over medium-low heat. Add the onions; cover and cook for 30 minutes, stirring occasionally. Uncover and add the vinegar. Stir until the vinegar evaporates, about 1 to 2 minutes.
In a large serving bowl, combine the potatoes, onions, remaining salt and pepper and Blue Cheese Dressing. Serve immediately, or cover and refrigerate. Bring to room temperature before serving.
In a small bowl, combine the mayonnaise, blue cheese, sour cream, horseradish, and pepper. Cover, and refrigerate until ready to use.
Yield: about 1 cup