Herb roast leg of lamb

Name: Herb roast leg of lamb
Ingredients: 75 g (3oz) fresh white breadcrumbs; Large handful fresh parsley, finely chopped; Small handful fresh mint leaves, finely sliced; Finely grated zest of 1 lemon; 2 garlic cloves, crushed; 1 medium egg; 50 g (2oz) pitted black or green olives, sliced; 2 kg (4½lb) whole leg of lamb (on the bone); About 5 fresh rosemary sprigs; 1 large onion, sliced into rounds (leave skin on); 3 tbsp. flour; 200 mL (⅓ pint) white or red wine; 300 mL (½ pint) chicken stock; Thick kitchen string