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.
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