Place the oil and butter in a roasting tin and heat at Mark 6 (200°C) 400°F for 5 minutes. Add the potatoes, turning them in the oil. Return to the oven for 15 minutes. Trim the fennel and remove the outer coarse leaves.

Cut each into quarters, lengthways. Add the fennel, olives and rosemary to the potatoes, turning them to coat.

Season with freshly ground black pepper. Return to the oven for 1/2 hour, basting occasionally, until the vegetables are crispy on the outside but soft inside.