Grate the yellow onion and squeeze out all excess liquid using a fine mesh strainer or cheesecloth. Discard or reserve the liquid for another use.
In a large mixing bowl, thoroughly combine the ground beef, drained grated onion, salt, black pepper, and saffron water.
Knead the mixture vigorously for 10 to 15 minutes until it becomes highly sticky, elastic, and holds together flawlessly.
Cover the bowl tightly with plastic wrap and refrigerate for at least 1 hour to allow the fats to firm up and flavors to meld.
Divide the chilled meat into equal portions. Mold each tightly around wide, flat metal skewers, pressing firmly with your thumb to create even ridges along the meat.
Preheat a charcoal or gas grill to high heat. Thread the tomatoes, green peppers, and red peppers onto separate skewers.
Place the meat and vegetable skewers on the grill. Turn the meat skewers frequently during the first few minutes to seal the outside, preventing the meat from falling off.
Continue cooking for 10 to 12 minutes, turning occasionally, until fully browned and cooked through.
Serve immediately on warm flatbread, garnished with sliced white onions, fresh basil leaves, grilled vegetables, and a generous sprinkle of sumac.