Preheat the oven to 200ºC, fan 180ºC, gas 6. Rub the lamb all over with the olive oil, and season well with freshly ground black pepper.
Put in a large roasting tin and scatter the shallots, turnips and garlic around the lamb. Sprinkle over the thyme and cover loosely with foil. Roast in the oven for 1 hour 50 minutes. Remove from the oven and let rest for 10 minutes. Carve 150g of lamb per person and serve with the roast veg.