Preheat the oven to 200ºC/gas mark 6. Rub the lamb all over with the olive oil, and season well.
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 the lamb and serve with the roast veg.