Beet and Sumac Salad

"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.