Combine all ingredients in a bowl and toss lightly. Serve on a platter next to meatballs, and garnish with sprinkled sumac.
*I like to grate the onion and garlic rather than chop them, as it makes for a smoother meatball, without chunks of onion and garlic. Grating the onion also releases it juices, which makes a nice moist meatball. I use a box grater for grating the onion, and a Microplane for the garlic.