Combine red onion, vinegar, sugar, and ½ teaspoon salt in a small bowl. Let sit at room temperature at least 30 minutes, or up to 2 days in advance, in the refrigerator. Drain before using.
Place cauliflower on a rimmed baking sheet and drizzle with 2 tablespoons olive oil. Season generously with salt and pepper and toss to coat. Transfer to oven and roast until golden and crisp at the edges, shaking pan occasionally, about 40 minutes. Remove from oven, sprinkle with 1 tablespoon sumac, and toss to coat.
Place garbanzo beans in a bowl and toss with remaining 2 tablespoons olive oil, remaining 1 tablespoon sumac, and zaatar. Season to taste with salt and pepper. Divide garbanzo beans, tomatoes, greens, parsley, olives, feta, and cauliflower among serving bowls. Serve with Lemon Tahini dressing.
For the Lemon Tahini Dressing:
In a medium bowl, whisk to combine tahini and lemon juice. Add olive oil, whisking until smooth. Add yogurt, lemon zest, and garlic. Season to taste with salt and pepper. Dressing can be kept covered in the refrigerator for up to one week.