Preheat oven to 400°F (200°C). Wash and scrub (or peel) potatoes, then slice them into wedges. Pat with a towel to dry, then transfer all to a bowl.
Toss potatoes with olive oil, lemon zest, salt and sumac. Lay them out on a baking sheet in a single layer and bake for 30 minutes. Give the potatoes a good stir, then increase the temperature to 425°F (220°C) and bake for another 10-15 minutes, until the potatoes are golden brown and crispy.
As the potatoes bake, make the dipping sauce: In a small bowl whisk tahini with water and lemon juice. They won’t come together right away, but as you keep stirring, the tahini and water will blend together into a creamy, smooth sauce. Mix in garlic powder, salt and cumin. Taste and adjust seasoning if needed. Keep sauce in the refrigerator until serving time.
When baked, take potatoes out of the oven and serve on a platter or in a shallow baking dish. Sprinkle with more sumac, fresh parsley, sea salt (flakey salt works too) and add a few lemon slices. Serve sauce alongside the potatoes, sprinkle with more cumin or sumac prior to serving.