Combine the chicken thighs, olive oil, ground cumin, paprika, turmeric, garlic powder, salt and black pepper in a bowl. Toss to coat and leave to marinate at room temperature for at least 15 minutes, or cover and chill in the fridge for up to 4 hours.
Rinse the basmati rice under cold running water until it runs clear. Melt the butter in a saucepan over a medium heat, then add the rice and ½ tsp salt. Stir for 1 minute.
Pour in the chicken stock (or water) and bring to the boil. Reduce the heat to low, cover and simmer for 15–18 minutes, or until the liquid is absorbed. Remove from the heat and leave, covered, for 5–10 minutes, then fluff the rice with a fork.
For the sauce, whisk together the mayonnaise, Greek yoghurt, white wine vinegar, lemon juice, minced garlic, caster sugar and ¼ tsp salt in a bowl until smooth. Gradually whisk in the water, 1 tbsp at a time, until the sauce reaches a pourable consistency.
Heat a frying pan over a medium-high heat. Add the marinated chicken in a single layer (cook in batches if necessary) and fry for 5–7 minutes, stirring occasionally, until browned and cooked through (internal temperature 74 °C). Remove from the heat.
Divide the rice between plates. Top with the spiced chicken, shredded iceberg lettuce and diced tomatoes. Drizzle over the garlic yoghurt sauce and serve with hot sauce to taste.