Heat the oven to 450°F with a rack in the lower-middle position. Line a rimmed baking sheet with kitchen parchment.
In a small bowl, whisk together the tahini and oil; set aside.
5 Tbsp Tahini, 1 Tbsp Olive oil
In a large bowl, whisk together the cornstarch, za’atar, 1/2 teaspoon salt, and 1/4teaspoon pepper. Add the sweet potatoes and toss to coat. Pour in the tahini mixture and rub the mixture into the potatoes; reserve the small bowl. Distribute in an even layer on the prepared baking sheet.
1 Tbsp Corn starch, 1/2 Tbsp Za’atar, 1 lbs Sweet potatoes, sliced into wedges
Roast until the potatoes are lightly browned on the bottom, about 12 minutes, rotating the baking sheet about halfway through. Remove from the oven and, using a thin metal spatula, flip each wedge. Roast until golden brown and a skewer inserted into the potatoes meets no resistance, about another 12 minutes, once again flipping the wedges and rotating the sheet halfway through.
In the reserved bowl, whisk together the yogurt, zest, and juice, tahini, za’atar, and 1/8 teaspoon each salt and pepper.
6 Tbsp Yogurt, full-fat, 1/2 tsp Lemon zest, 1 Tbsp Lemon juice, 1-1/2 Tbsp Tahini, 1/2 tsp Za’atar
Transfer the potatoes to a platter. Serve with the yogurt-citrus sauce.
Optional: Garnish with fresh parsley