Toss the chicken thighs with the olive oil in a roasting pan that fits them all comfortably and in a single layer with space between them. Make the za'atar simply by mixing the sumac, thyme, sesame seeds, and kosher salt together and scatter it over the chicken, pressing it into the meat. Cover and refrigerate for 12 – 24 hours.
Let the chicken come to room temperature for at least one our—but up to two—before preheating the oven to 450°. Roast the chicken for 20 minutes—the chicken should be golden-brown around the edges and the top almost brick-red. Set the chicken aside to cool slightly before roughly chopping or shredding the meat. Keep the oven on.
For the tomato-mint salad… Simply enough, cut the tomatoes in half, remove the stem portion and scoop their guts out and discard. Chop the tomatoes into pretty large chunks and transfer to a large bowl.
If the tomatoes don’t taste absolutely perfect then sprinkle over a little bit of the salt and sugar and set aside for a few minutes.
At the last possible minute, thinly slice the green onions, very, very, very roughly chop the mint and toss together, along with the lemon, oil and any remaining sugar and salt.
Put the pitas in the oven for just a few moments to crisp them a little (otherwise the bread can get soggy from the tomatoes). Remove the pitas and turn on the broiler.
Scatter the pitas with the chicken and tomato-mint salad. Adorn the top of the pitas with slices of the halloumi. Slide them under the broiler for just a few moments, keeping a close on them—you’re looking for the cheese to soften and blister a bit, but take care not to burn anything.
Remove form the oven, slice, and serve immediately or at room temperature.
Feel free to grill both the chicken and the flatbreads in the summer—it’s particularly delicious this way.
You can up the quantities of this very easily to serve for a huge feast as, say, an appetizer or part of huge spread. For this, I would probably just buy a whole chicken (probably figuring about 5 - 6 ounces per person) and either break it down into portions or spatchcock it and cook it this way.