Either use a cheese grater or food processor, but grate the onions and drain in a sieve set over a bowl for at least 2 hours. Squeeze the grated onions to release any leftover juice and place in a large bowl. Discard onion juice or save for soup or other use.
To the bowl add the ground beef and lamb, shredded onions, garlic, egg, bread crumbs, parsley, kosher salt, pepper, turmeric, and paprika, and mix well. Divide meat mixture evenly into eighths and form the meat into 8 large flat wide metal skewers, molding and flattening meat around skewer to make each kebab about 8" long. If not using skewers, shape meat into eight 8" hotdog/sausages. Set kebabs aside. This can be grilled right away or allowed to marinate overnight.
Heat a small skillet over medium heat for 2 minutes. Remove the skillet from heat, add saffron threads, and swirl threads around pan for 30 seconds to lightly toast. In a small bowl crush the saffron and then add 1 tablespoon boiling water and stir in butter.
Brush the kebabs with saffron butter. At this point you can cover and refrigerate until ready to grill, or place the skewered kebabs on 350°F grill and cook until browned on each side and cooked through, 3–4 minutes per side. Brush the kebabs with remaining saffron butter and sprinkle with a little ground sumac.
Slide kebabs off skewers and serve with steamed rice and a squeeze of fresh lemon.