Place racks in center of the oven. Preheat the oven to 425 degrees Fahrenheit (or 400 degree, see notes). Line two large baking sheets with parchment paper.
Cut the sweet potatoes into ¼ inch sticks. (Optional: salt and allow to rest for crispier fries - see notes.)
Place the sweet potato sticks in a large mixing bowl and drizzle with the tahini. Using your hands, toss the sweet potato sticks thoroughly with the tahini, rubbing it over all of the sticks until well coated. Sprinkle with the Za'atar seasoning and salt, and toss again until evenly coated.
Spread the sweet potato fries evenly over the baking sheets in a single layer. Bake at 425 degrees for 10 minutes, turn over, and bake for another 10 minutes, or until the fries are well browned and a bit crispy. Check occasionaly to make sure they're not getting too dark. (See notes)
To make the Oil Free Aioli
Place all of the ingredients in a food processor (or a blender, if making the cashew version), and process until smooth and creamy, about 1 minute. For the best flavor, allow to rest in the refrigerator for at least 30 minutes.
Notes
For slightly crispier fries, salt the sweet potato sticks, allow them to rest for 30 minutes, and pat them dry, before seasoning and cooking.
Cooking at 425 for 20 minutes results in well browned fries, but thin tips may become dark. If you prefer a lighter, less browned fry, cook at 400 degrees for 25-30 minutes, turning them over after 15 minutes. If choosing this method, I recommend drawing out water by salting the fries (see note above), otherwise, they don't brown well.
Kala namak, or Indian black salt, is optional but highly recommended. It has a mild sulfur flavor (like eggs) that makes the aioli taste more like 'real' mayonnaise.