In a large bowl, combine ground beef, ground lamb, grated onion, salt, pepper, and turmeric. Mix well by hand for 2–3 minutes until sticky.
Cover and refrigerate for at least 1 hour or overnight.
Divide meat mixture into 8–10 equal portions. Mold onto flat skewers into 6–8-inch long logs, squeezing to shape and secure.
Preheat grill to high heat. Oil grates well.
Grill kabobs 4–5 minutes per side or until cooked through and lightly charred.
Serve over rice with grilled tomatoes. Garnish with parsley and sumac if desired.