Char the onions on a gasflame. After the skin blisters, let it rest for 15 minutes and then peel.
Mix all the ingredients and let them sit for 10 minutes before serving.
Tip for Charring with Direct Heat:
Place the onions or garlic on a very hot griddle and turn them around till all the skin is blistered and black. Then leave them to cool (some prefer to keep them in a plastic bag) and then peel. It gives a lovely smoky flavour and a juicy texture to the onions.