Cut the tomatillos in half and roast them with the onion slices and garlic cloves over a barbecue fire. (Alternatively, you can broil them or brown them in a heavy, dry skillet.) The tomatillos and onions should be lightly-browned, and the garlic should be soft.
Cool the garlic slightly, then peel it, cutting off any scorched parts as they will give the salsa a bitter flavor.
Put the tomatillos, onion and garlic in a food processor with the remaining ingredients. Process until the mixture is chunky, not smooth. (If the salsa is too dry, add a little more olive oil, lime juice or water.