Cook the chicken and onions: Position a rack in the middle of the oven and preheat to 350 degrees.
2. ### Step 2
In a large skillet over medium-low heat, combine the olive oil, onions and 1 teaspoon of the salt and cook, stirring occasionally, until the onions soften considerably but do not brown or caramelize, 30 to 40 minutes.
3. ### Step 3
Meanwhile, in a large saucepan over medium heat, combine the chicken, 1 teaspoon of the salt, the bay leaf, allspice berries and enough water to cover. Bring to a bare simmer, then reduce the heat to low and cook until the chicken registers 165 degrees on an instant-read thermometer, 15 to 20 minutes. Remove from the heat, then drain and rinse the chicken. Transfer to a bowl, cover and set aside until cool enough to handle, then shred the chicken using your hands or two forks.
4. ### Step 4
Arrange the pita squares on a large, rimmed baking sheet, transfer to the oven and toast for about 15 minutes, or until dry and crisp and starting to darken, stirring occasionally. (If using store-bought pita chips, skip this step.)
5. ### Step 5
When the onions have softened, add the shredded chicken, sumac, cumin, pepper and cinnamon and stir to incorporate. Continue to cook, stirring frequently, until the flavors meld, 3 to 5 minutes. Taste, and season with the remaining 1 teaspoon of salt, if desired. Add the pine nuts, and toss or stir to combine.
6. ### Step 6
Make the yogurt sauce: While the onions and chicken cook, in a large bowl, whisk together the yogurt, lemon juice, tahini, garlic and salt until fully combined. Set aside until ready to assemble.
7. ### Step 7
Assemble the dish: Place the toasted pita on the bottom of a large, rimmed serving platter and top with the chicken-and-onion mixture. Evenly spoon the yogurt sauce over, smoothing it out if necessary. Sprinkle with your chosen combination of garnishes and a drizzle of olive oil, and serve.
