Arrange the rack at the top of the oven, then preheat the fan-assisted oven to 220 °C / 430 °F or the conventional oven to 240 °C / 465 °F. Line a baking sheet with parchment paper. If you're using wooden skewers, cover them with water in a shallow pan to soak them for 10 minutes. This will prevent burning.
marinate the meat
Cut the chicken thighs in half to get 12 halves. Add to a bowl along with roughly chopped onion, greek yogurt, lemon zest, lemon juice, and olive oil. Season with salt, pepper, and gyros spice mix. Stir to combine and coat the meat and set aside for 15 minutes (or store in the fridge overnight).TipIf you can't get a gyros spice mix in your country, make a mixture of onion powder, paprika powder, garlic powder, oregano, and rosemary. Use two tablespoons of the mix.
thread the meat
Use two metal (or wooden) skewers long enough to sit across the prepared baking sheet. Fold each piece of meat in half to create a pocket. Add a piece of marinated onion into the pocket, then thread the meat onto the skewers tightly together. Put the skewers crosswise on the prepared baking sheet. The meat shouldn't touch the baking sheet.
Roast the meat in the preheated oven on the highest rack for 25 - 30 minutes in a fan-assisted oven at 220 °C / 430 °F or in the conventional oven at 240 °C / 465 °F. Turn the skewers halfway through to roast them all the way around. Change the oven temperature to high broil. Broil for 5 minutes or until they are cooked and tender inside, while slightly charred and grilled outside.
Carefully remove the meat from the skewers and cut it into thin slices. Spread a tablespoon of tzatziki sauce over warm pita bread. Add the lettuce, sliced cucumber, and sliced tomato. Place the meat on top and drizzle with additional tzatziki sauce. Wrap, and serve.