Remove the lamb from the fridge 1 hr before cooking. Heat the oven to 200C/180C fan/gas 4. Use the tip of a knife to make 20-25 small slits in the lamb, then set aside in a roasting tin. Blitz the watercress, garlic and anchovies in a food processor until smooth, then add the butter and pulse to combine.
Season the lamb, then spread over half of the butter mixture, pushing into the slits as well. Break the rosemary sprigs into pieces and tuck into the slits. Season, cover with foil and cook for 1 hr 20 mins for medium (a thermometer should read 55C), or 1 hr 40 mins, if you prefer the meat well done. Uncover the lamb halfway through cooking and spread over the remaining butter, then put back in the oven uncovered for the remaining time.
Meanwhile, blitz the sourdough in a food processor to make coarse crumbs. Toss with the oil and parmesan and spread out flat on a baking sheet. Put in the oven underneath the lamb for the last 20 mins of cooking, or until lightly golden and crunchy. Leave the lamb to rest, covered, for 30 mins, then slice and sprinkle over the crumbs.