Preheat oven to Gas 4, 180ºC, 350ºF.
In a pan heat the oil, add the onion and garlic. Lightly cook until beginning to soften. Add the olives, anchovies and rosemary, continue to cook for 1-2 minutes. Take the boneless leg of lamb and open, place flattish on a chopping board.
Season the meat and then lay the spinach on top. Spoon over the onion mixture and roll up the joint, if spinach falls out just push it back in. Secure with string or use some long metal skewers.
Place in the bottom of a large roasting pan. Pour over the wine, cover with foil and cook covered for half the calculated cooking time (25 minutes per 450g/½kg (1lb) plus 25 minutes).
Remove the foil (add a splash more wine), baste the joint and cook for the remaining time. Allow the joint to rest for 10 -15 minutes before serving.
Serve with seasonal steamed green vegetables. (You can roast potatoes around the joint - they will absorb some of the liquid but be crispy on top).