For the seitan: Pulverize the onions in a food processor until the onion juice is rendered. (If you don't have a food processor, place the onion in bowl and mash with the bottom of a jar.) Remove the onions and juice to a ceramic dish. Add the garlic, oil, lemon juice, coriander, cinnamon, cumin, curry, oregano, pepper, salt and turmeric and mix well. Slice the seitan into paper-thin strips. Layer the seitan slices in the marinade, cover and refrigerate overnight.
Remove the slices from the marinade, taking care to scrape off most but not all of the onion. Fry the slices in olive oil in a skillet over medium-high heat until crispy, about 3 minutes on each side. Remove to a plate and turn off the heat.
For assembly: Warm the pita breads in the same skillet with the heat off. Open the pita pocket by slicing off one edge and gently spreading the two halves apart. Stuff with seitan strips and veggies, and top with yogurt and mint.