Melt the butter in an ovenproof dish and put in the cleaned potatoes. Bake in a hot oven,
Meanwhile, put the onions into a wide bowl. Make a dressing by crushing the garlic in the salt and pepper, stirring in the mustard and then the vinegar and oil. Pour over the onions and leave to marinade. When the potatoes are cooked, add them immediately to the onions and serve at once.