Preheat the oven to 425°F.
On a large rimmed baking sheet, toss together the chickpeas, onion, garlic, olive oil, salt, cumin, coriander, and cayenne. Roast until the onion and garlic have started to soften, 15 to 20 minutes.
Scatter the pita pieces on top, sprinkle them with sumac, and continue roasting until the pitas are crisp and the onion and garlic are very soft, 8 to 10 minutes.
Remove from the oven, fish out the garlic to use in the dressing, and let everything cool to room temperature.
While the chickpea mixture is cooling, make the dressing. In a small bowl, mash the reserved roasted garlic with a fork, then whisk in the lemon juice, tahini, ¼ cup water, and salt. Taste and add more salt if needed.
To assemble the salad, toss the chickpea mixture with the romaine, parsley, tomatoes, and pickles. Drizzle with the dressing and serve immediately.
