Place lamb cutlets on a large plate and sprinkle with flaky salt and grind over black pepper. Sprinkle with lemon zest and juice, thyme leaves and olive oil and rub it all into the lamb cutlets with your fingertips. Cover and set aside for 10 minutes.
Make the yoghurt sauce; combine all the ingredients in a bowl and mix. Set aside.
Heat a large frying pan or grill pan over a medium-high heat. Place in the lamb cutlets and cook for 3 minutes on each side for medium lamb. (Cooking time will depend on thickness of cutlets).
Remove from the frying pan to a serving plate, drizzle with extra olive oil and sprinkle with the chopped parsley or coriander and chilli, if using. Serve with the yoghurt sauce.