Mix the ground spices with the yogurt and a squeeze of lemon and coat the lamb all over. Leave to marinate while you prepare the potatoes.
Toss the potatoes with the oil, mustard seeds, a little salt and lots of black pepper. Tip into a shallow roasting tin and cook for 25 minutes.
While the potatoes are cooking, remove the lamb from the marinade and put it on a nonstick baking sheet, squeeze over half of the lemon and grill under a very hot grill for 5 minutes on each side or until browned and cooked through. Once the potatoes are cooked squeeze over the other half of the lemon.
Serve with greens or salad.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.