Preheat the oven to 200C/400F/Gas 6.
For the lamb, place the walnuts, chives, garlic, breadcrumbs and butter into a small food processor and blend until the mixture resembles breadcrumbs.
Rub the lamb steak with one tablespoonful of the oil and press the breadcrumb mixture onto one side of the lamb steak.
Heat the remaining oil in an ovenproof frying pan and fry the lamb crust-side up for 3-4 minutes. Transfer the pan to the oven and cook for 8-10 minutes, or until the crust is crisp and golden-brown and the lamb is cooked to your liking.
For the potatoes, cut off a slice along the length of each potato to make one flat side. Make 3-4 vertical cuts into the uncut side of each potato, almost to the base. Drizzle with olive oil and season well with salt and freshly ground black pepper.
Place the potatoes onto a baking sheet, transfer to the oven and roast for 10-15 minutes, or until golden-brown and crisp.
To serve, place the lamb onto a serving plate with the potatoes.