1: Butterfly the steak by cutting it through the short side to the opposite side. Open the steak and arrange on a cutting board. Using the back (dull side) of a heavy chef’s knife, pound the steak along its length at intervals of about 1/4 inch. Arrange in a single layer in a nonreactive baking dish. (Alternatively, place the meat in a large resealable plastic bag.)
2: Using a grater or the grating disk of your food processor, finely grate the onion. Place the onion in a fine sieve over a medium bowl. Squeeze the onion (or press it with a spoon) until it releases its juices. (Reserve the onion bits for another use.)
3: Soak the saffron in 1/2 cup of hot water for 30 minutes.
4: Add half the saffron-infused water to the onion juice. (Reserve the remaining half for basting.) Add 1 teaspoon of salt, 1/2 teaspoon of pepper, and the olive oil. Stir to combine. Pour the marinade over the steak; turn the steak to coat. Cover (or seal the plastic bag) and refrigerate for at least 8 hours and up to 24.
5: Remove the steak from the marinade and pat dry with paper towels. Carefully run a flat metal skewer through each piece of steak, starting from the short side and running the length of the steak. (Alternatively, use two long bamboo skewers; run them parallel to each other to prevent the steak from spinning when it’s turned.)
6: Set up your grill for direct grilling and heat to high. Brush or scrape the grill grate clean and oil it well with vegetable oil.
7: Season the steak kebabs with salt and pepper. Arrange on the grill grate and grill for about 2 minutes per side, or until cooked to your liking, brushing once or twice with the remaining saffron-infused water.
8: Transfer to a platter or plates. Serve with rice, grilled tomatoes and lemon halves, and diced onion. Sprinkle lightly with ground sumac, if using, and chopped parsley. Warmed lavash makes a nice accompaniment.
Note:: If desired, you can substitute center-cut filet mignon (butterfly 3 inch pieces), sirloin, or skirt steak.