Preheat the oven to 200°C.
Wipe the meat then make lots of 5mm deep crosses on the surface . Put 1 slice of garlic, half a slice of anchovy and rosemary piece into each incision.
Peel the onions, cut in half and place in a baking tray.
Rub the meat with olive oil and season with salt & pepper, then place in a roasting pan. Pour over the port.
Roast for about 90 minutes, depending on how well cooked you like your lamb. Transfer it to a plate, cover and let it rest for 15 minutes while you make the gravy.
The gravy: Discard the onion and put the roasting tray on the hob. Add some stock, butter (optional), some more port or vegetable stock and reduce it to a thick, silky sauce.
Carve the lamb and serve it with your chosen vegetables and, of course, the lovely gravy.