Combine the sumac, garlic and olive oil in a dish, add the lamb and toss to coat. Cover and place in the refrigerator for two hours.
Place the yoghurt together with 2 tsp lemon juice into a small dish and mix to combine. Season with salt and pepper and set aside.
Steam the chat potatoes for 15 minutes, adding the beans for the final 7 minutes, seasoning with a little salt to taste.
Heat a non-stick frying pan over medium high heat, add the lamb and cook for 8-10 minutes, turning half way through the cooking, until cooked to your liking. Remove the lamb from the pan and set aside to rest.
Deglaze the pan with the muscat and keep warm.
Arrange the potatoes, beans and cherry tomatoes on individual serving plates, top with the lamb and drizzle over the muscat sauce. Scatter over the olives and capers and serve with a flourish of the yoghurt sauce.