Make ahead: In a medium-size bowl, combine onion slices, sumac, and salt. Cover and chill for several hours.
For the vinaigrette: Preheat oven to 400. Place lemon halves (cut side down) in a baking dish, alongside unpeeled garlic cloves. Roast for 25—30 minutes, until lemon tops have begun to brown. Remove from oven and set aside to cool. Then squeeze out roasted garlic into a small prep bowl. Mash, then squeeze in lemon juice. Add honey and balsamic, then whisk in olive oil. [Vinaigrette found on sandwichsunday.com]
To make the salad, combine chickpeas, onions, mint, and parsley in a large bowl. Pour in vinaigrette, olive oil, and pomegranate molasses. Toss and serve.