1In a large bowl, whisk the yogurt, juice of 1 lemon, lemon zest, 4 cloves of the garlic, olive oil, cumin, coriander, paprika, turmeric, cinnamon, allspice, cayenne, 1 1/2 tsp of the salt, and the black pepper into a thick paste.
2Add the sliced lamb and toss until every strip is coated. Let it marinate at room temperature for 30 minutes, or covered in the fridge for up to 24 hours.
3Heat the oven to 425°F with a rack in the upper third. Line a large sheet pan with parchment.
4Scatter the red onion over the pan, then spread the lamb on top in a single layer, leaving a little space between strips so they roast rather than steam.
5Roast 18 to 22 minutes, stirring once halfway, until the lamb is cooked through and the edges are browned. For extra char, switch to broil for the last 2 to 3 minutes and watch closely.
6While the lamb roasts, make the sauce: whisk the tahini with the juice of the second lemon, the remaining 2 cloves of garlic, and 1/4 tsp salt. Whisk in cold water a tablespoon at a time, about 1/4 cup, until it is pourable and pale. Taste and adjust the salt and lemon.
7Toss the cucumber, tomatoes, and half the parsley with the sumac and the last pinch of salt for a quick salad.
8Spread the warm pita on a large platter. Pile the roasted lamb and onions in the center, spoon the salad alongside, drizzle everything with tahini sauce, and finish with the remaining parsley.