Heat the oven to 425 degrees. On a large rimmed sheet pan, drizzle the mushrooms and red onion with 3 tablespoons oil. Sprinkle with cumin, coriander, paprika (if using), 1 teaspoon salt and ½ teaspoon pepper; toss to coat. Arrange in an even layer and roast until tender and browned, about 20 minutes. Add the pitas directly to the oven rack to warm during the last 5 minutes of cooking.
Meanwhile, toss cabbage with remaining 2 teaspoons oil in a medium bowl; toss to coat. Season generously with salt and pepper. In a small bowl, stir together yogurt and turmeric; season with salt and pepper.
To serve, slather yogurt over warm pitas. Top with cabbage, mushroom mixture and herbs, and serve immediately. (I went freestyle here and slathered the yogurt on top, which made for a messier sandwich).