Preheat the oven to 190°C / fan 170°C / gas 5. Put the ciabatta, herbs, garlic and lemon zest in a food processor. Blitz until finely chopped. Drizzle in the melted butter and blitz again.
Put the leg of lamb in an ovenproof dish or roasting tin and rub the mustard over it, then cover with the herb mixture, pressing down well.
Cook for 50 minutes. Remove and cover with foil, then return to the oven for a further 25 minutes. Leave to stand for 15 minutes before carving into slices and serving.