Place the yogurt, saffron water, grated onion, lemon juice, salt, and pepper in a bowl and mix together. Add the Chateaubriand Steaks. Place the steaks into the marinade and turn over once or twice to coat each steak completely in the marinade. Cover and refrigerate for 4 hours before cooking. Preheat grill to high temperature. Remove the marinated steaks from the refrigerator and scrape off excess marinade, and set aside on a clean plate while preparing the basting sauce. For Sauce: In a small bowl mix together saffron water, lemon juice, and melted butter. Place steaks on grill. With a brush, baste the steaks with basting sauce while cooking. Cook according to your preference. Take steaks off grill and sprinkle equal amounts of powdered sumac on the top (scored) surface of the steak. Serving Suggestion: Serve with Basmati Rice and Grilled Vegetables.