Add all ingredients for the sumac onions to a bowl and stir well. Cover and refrigerate until using (at least 30 minutes or up to 3 days).
Add all ingredients for the kebabs to a large bowl. Use your hands to combine, being careful not to over-mix.
Slightly wet your hands, and shape the meat onto skewers. Brush both sides of each skewer with olive oil.
Preheat a grill to medium-high. Once hot, add the kebabs and grill until they’re charred on the outside and cooked to your desired level of doneness inside, rotating the skewers as necessary. (The exact cooking time will vary based on your skewer size. I used 3 large skewers and it took about 15 to 18 minutes total.)
Serve the kebabs hot along with sumac onion and anything else you like.
