Vegetable Shawarma

1. Whisk together the pumpkin spice blend, chili powder, dried oregano, cumin, 2 teaspoons garlic powder, 2 teaspoons salt and 1 1/2 teaspoons black pepper in a large bowl. Heat 1/2 cup of the olive oil in a small saucepan over medium heat until it starts to shimmer, 2 to 4 minutes. Pour the hot oil over the spices, then whisk to combine. Set aside to cool slightly.
2. Peel the celery root, then cut off both ends so that they are flat. Cut the celery root in half crosswise. Cut one half of the celery root into 3/4-inch chunks and leave the remaining second half whole. Put all the celery root into the oil and spice mixture. 
3. Cut the butternut squash into 2 pieces where the long neck and squat bottom meet. Peel both pieces, then use a spoon to remove the seeds from the bottom part of the squash and cut into 3/4-inch chunks. Use a mandoline to slice the neck into fifteen 1/8-inch-thick round discs. Cut any remaining butternut squash into 3/4-inch cubes. Add to the bowl of oil and spices. Cut 1 inch from the smaller ends of the potatoes, then cut them into cubes. Use a mandoline to slice the remaining Yukon gold potatoes into fifteen 1/8-inch-thick discs, then cube any remaining potatoes. Place the potatoes into the spice mixture. Using the mandoline, slice the red onion into fifteen 1/8-inch-thick rounds. Put the onion rounds into a separate plastic storage container (try to keep the rounds as intact as possible). Use your hands to toss the vegetables in the oil and spices until everything is completely coated. Cover with plastic wrap and place in the refrigerator along with the onions for 2 hours or up to overnight. 
4. Meanwhile, combine the preserved lemon, Greek yogurt, parsley, remaining 1/2 teaspoon garlic powder, remaining tablespoon olive oil, 1 tablespoon water and a large pinch of black pepper in a medium bowl. Whisk to combine, then season with additional salt if needed. Refrigerate until ready to serve.
5. Move an oven rack to the very bottom position and remove all other racks from the oven. Preheat to 375 degrees F. 
6. Remove the marinated vegetables and onions from the refrigerator. Use a slotted spoon to transfer the vegetables to a large baking sheet (leaving any extra marinade behind in the bowl). Put the celery root half in the center of a medium roasting pan (the smaller end should be on the bottom). Insert the wooden skewer deeply into the center of the celery root, pointed-side-up. 
7. Layer 1 butternut squash disc and 1 potato disc on the skewer. Lightly coat 1 onion round in the remaining marinade and layer it on top of the potato disc. Continue layering in this manner until all veggie discs and onion rounds have been used, and being careful to keep the vegetables as centered as possible on the celery root. Use your hands to lightly press down on the fully assembled shawarma so that the vegetables are compact. Use a pair of kitchen shears to trim the wooden skewer, leaving only 1 inch at the top. 
8. Scatter the remaining vegetables around the celery root base, then bake, stirring the loose vegetables around the pan a couple of times during the cooking, until the vegetables are golden brown and tender and you can insert a cake tester or thin wooden skewer into the center of the vegetables without any resistance, 1 hour 15 minutes to 1 hour 30 minutes. 
9. Allow to rest for 10 minutes. Use a sharp knife to thinly shave the vegetable shawarma and serve with the Greek yogurt sauce, lettuce, tomatoes and warm pitas. 
10. Special equipment: a mandolin; an extra-thick 12-inch wooden skewer

DIY Pumpkin Spice Blend:

1. Combine cinnamon, ginger, cardamom, nutmeg, allspice and cloves in a bowl and mix well. Store in an airtight container for up to 3 months.