Crush the onion with the sumac and salt using a pestle and mortar or back of a fork. Combine with the remaining ingredients in a medium mixing bowl. Set aside for at least 1-3 hours for flavor to enhance. Adjust seasoning if necessary just before serving. Transfer to a pretty serving bowl or platter and serve with chips, slices of toasted French baguette (<--yum!), kebabs, pita crisps, etc.