Make the marinade. Combine Greek yoghurt, tikka masala spice blend, minced garlic, grated ginger, lemon juice, olive oil, and salt in a large bowl. Stir well until smooth. Add the sliced chicken and toss until every piece is evenly coated.
Marinate the chicken. Leave the chicken to marinate for at least 20 minutes at room temperature. For deeper flavour, cover and refrigerate for 2–4 hours.
Cook the chicken. Heat a grill pan or large skillet over medium-high heat. Add the chicken in a single layer — do not overcrowd the pan. Cook for 4–5 minutes per side until golden, lightly charred at the edges, and cooked through. Set aside to rest for a couple of minutes.
Warm the flatbreads. Place flatbreads in a dry skillet for 1–2 minutes per side, or wrap in foil and warm briefly in the oven. They should be soft and pliable.
Prep the toppings and yoghurt sauce. Halve the tomatoes, slice the cucumber and red onion, and shred the lettuce. In a small bowl, mix together the Greek yoghurt, lemon juice, garlic, herbs, salt, and pepper for the sauce. Taste and adjust seasoning.
Assemble. Spread a generous layer of yoghurt sauce over each warm flatbread. Layer on the shredded lettuce, cooked chicken tikka, cherry tomatoes, cucumber, and red onion. Scatter fresh coriander over the top and finish with another drizzle of yoghurt sauce.
Serve immediately. Fold or slice the flatbreads and bring them to the table while everything is still warm.