1) Preheat the oven to 220C/Gas 8. Line a large shallow roasting tray with non-stick aluminium foil.
2)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 tray. Bake until the potatoes are tender and lightly browned, about 35 minutes, stirring occasionally. Remove from the oven and cool.
3) 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 1-2 minutes.
4) In a large serving bowl, combine the potatoes, onions, remaining salt and pepper, and blue cheese dressing.
BR> Serve the salad immediately, or cover and refrigerate. Bring to room temperature before serving.
Blue cheese dressing:
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