"[This] Middle Eastern winter salad with a creamy, nutty taste and a delicious bite of onions, flavored with sumac and lemon," writes Jake Tilson, "[is] a great addition to a mezze supper." Sumac, a purple-hued spice that's ground from the dried berries of the sumac tree, has a sour, almost astringent bite. It's used widely all over the eastern Mediterranean as a flavoring and souring agent, in such preparations as hummus and the ubiquitous sliced onions that accompany grilled meat dishes. Cook 3 unpeeled large beets in a large pan of gently boiling water until tender, 30–40 minutes. (Add a drop or two of red wine vinegar to help set the beets' color.) Drain the beets and let cool. Peel the beets, then cut them into quarters and thinly slice. Put the sliced beets, 2 tsp. ground sumac, 2 handfuls arugula leaves, and 1 thinly sliced large red onion into a serving bowl. Mix 1 cup whole milk yogurt, 1–2 tbsp. lemon juice, 2 tsp. tahini, 2 crushed cloves garlic, and salt and pepper to taste in a bowl. Pour dressing over the salad and toss gently. Serve with a flat bread, such as lavash, and marinated olives, if you like. Serves 6.