Vegetable Shawarma

Name: Vegetable Shawarma
Ingredients: 2 tablespoons DIY Pumpkin Spice Blend, recipe follows; 2 teaspoons chili powder; 2 teaspoons dried oregano; 1 1/2 teaspoons ground cumin; 2 1/2 teaspoons garlic powder; Kosher salt and freshly ground black pepper; 1/2 cup plus 1 tablespoon extra-virgin olive oil; 1 extra-large celery root (about 4 pounds); 1 medium butternut squash (about 3 pounds); 2 large Yukon gold potatoes (about 1 pound); 1 large red onion (about 9 ounces); 1 small preserved lemon, finely chopped; 1 1/2 cups full-fat plain Greek yogurt; 1/4 cup fresh flat leaf parsley leaves, roughly chopped; 1 large head romaine or 2 small romaine hearts, thinly sliced; 2 large Roma tomatoes, thinly sliced; 8 large pita pockets, warm; 1/4 cup ground cinnamon; 2 tablespoons ground ginger; 1 tablespoon ground cardamom; 1 tablespoon ground nutmeg; 1 teaspoon ground allspice; 1 teaspoon ground cloves