Step 1

Stir the oil, garlic, salt, pepper, cumin, cinnamon, and turmeric together in a large bowl to form a paste. Add the chicken and toss to coat; let marinate at room temperature while the oven heats, about 20 minutes. Arrange a rack in the middle of the oven and heat to 425°F.

Step 2

Line a rimmed baking sheet with parchment paper or oiled aluminum foil. Place the chicken on the baking sheet in single layer. Roast until browned and cooked through, 25 to 30 minutes. Meanwhile, make the dressing and begin to assemble the salad.

Step 3

Whisk the tahini, lemon juice, oil, 2 tablespoons of the water, salt, and black pepper together in a small bowl until smooth. If the dressing is too thick to pour, whisk in the extra tablespoon of water.

Step 4

Place the onion and water in a small bowl and set aside for 10 minutes to make the onion less potent. Drain and discard the liquid.

Step 5

Place the lettuce in a large bowl, drizzle with half the dressing, and toss to coat. Divide the lettuce, tomato, cucumber, and red onion among 4 plates or shallow bowls. When the chicken is ready, let rest 5 minutes, then thinly slice crosswise and divide among the salads. Drizzle with the remaining dressing, sprinkle with sumac if using, and serve.