Preheat the oven to 200C/400F/Gas 6.
For the lamb, rub the lamb with the oil, fresh rosemary and lemon zest and season well with salt and freshly ground black pepper.
Place the lamb into a hot pan over a medium heat and fry for 4-5 minutes on each side, or until cooked through. Squeeze over the lemon juice on each side during cooking.
For the roasted shallot, place the shallot into a small roasting tin, drizzled with olive oil and place into the oven to roast for 10-15 minutes, until browned and soft.
For the sweet potato mash, place the sweet potato into boiling salted water and boil for 10 minutes, then drain over a colander. Return the sweet potato to the pan and mash, stirring in the olive oil and salt and freshly ground black pepper, to taste.
To serve, place the lamb onto a pile of sweet potato mash with the roasted shallot on the side.