Pre-heat oven to 180c. Mix the olive oil, thyme, zest and lemon juice. Season and spoon half the mixture over the lamb and toss the other half through the vegetables.
Place lamb into a roasting dish and bake for 15 minutes before adding the tossed vegetables and cooking for a further hour. Add the beans and cook a further 5 minutes.
Rest the meat under foil in a warm place.
To serve, slice the lamb and serve with the vegetables, scattered with fresh thyme leaves and lemon zest.