Drizzle the lamb steak with olive oil and season with salt and pepper.
Mix all spices in a small bowl and coat the lamb thoroughly.
Chargrill the lamb in a hot griddle pan for 4 to 5 minutes per side then let it rest.
Squeeze excess moisture from the grated sweet potato using a clean tea towel.
Combine sweet potato, apple, salt, pepper, and half of the butter in a bowl.
Fry the mixture in the remaining butter and oil for 4 to 6 minutes per side until crisp.
Serve the lamb steak on top of the sweet potato rosti.