Preheat the oven to 200°C fan. In a large bowl, mix the Greek yoghurt, lemon juice, garlic, oregano, onion granules, smoked paprika, cumin, salt and plenty of black pepper. Add the chicken thighs and mix until evenly coated. Spread onto a lined baking tray and roast for 30 minutes until cooked through and lightly caramelised around the edges.
At the same time, cook the fries according to the packet instructions until crisp and golden.
To make the tzatziki, combine the Greek yoghurt, grated cucumber, garlic, mint and lemon juice. Season well with salt and black pepper.
Once the chicken is cooked, shred it with two forks or slice with a sharp knife. Toss the shredded chicken back through all the juices left on the tray.
Heat your flatbreads according to the packet instructions. Spread a generous spoonful of tzatziki over each warm flatbread. Top with shredded lettuce, diced tomatoes, crispy fries, plenty of the shredded chicken and pickled red onions.
Finish with another spoonful of tzatziki and a squeeze of lemon. Fold up and enjoy!