In a large bowl, combine the onion, sumac and ¼ teaspoon of the salt. Use your hands to rub the sumac and salt into the onion slices, massaging them until the onions wilt, about 5 minutes.
Add the chickpeas, tomatoes, bell pepper, scallions and parsley, and stir to combine.
In a small bowl, whisk together the olive oil, lemon juice, the remaining ½ teaspoon of salt and the pepper until combined. Taste, and season with more salt and pepper as needed.
Pour the dressing over the salad and toss to mix well. Sprinkle with more sumac and serve.