For the curried lamb steak, drizzle the lamb steak with olive oil and season with salt and freshly ground black pepper.
Mix all the spices together in a small bowl, then dredge the lamb in the spices to coat.
Heat a griddle pan and chargrill the lamb for 4-5 minutes on each side, or until cooked to your liking. Remove from the pan and set aside to rest in a warm place.
For the rösti, place the grated sweet potato onto a clean tea towel and squeeze out the excess moisture.
Place the sweet potato into a bowl with the apple and half of the butter, season well with salt and freshly ground black pepper and mix well.
Heat the remaining butter and the oil in a small non-stick frying pan and press the sweet potato mixture into the base of the pan. Fry for 4-6 minutes on each side, or until crisp and golden-brown.
To serve, place the rösti onto a serving plate and top with the curried lamb steak.