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