In a small bowl, add the rosemary, oregano, thyme, garlic, mustard, salt, pepper and lemon zest. Mix well.
Remove any string from the lamb and make diagonal cuts into the surface of the joint.
Rub the rosemary mix all over the lamb joint, getting plenty of the rub into the diagonal cuts.
Add the potato, carrot, onion, pepper, tomatoes and garlic to the slow cooker pot and season well.
Nestle the lamb between the vegetables.
Pour in the lemon juice and water into the pot. Cover with a lid and cook on low for 9-10 hours, until the lamb is tender.
Preheat the oven to 200°C.
Remove the lamb from the slow cooker pot and shred with two forks.
Add the shredded lamb and vegetables to a large baking tray and cook for 20 minutes to crisp up the lamb around the edges. Serve with an accompaniment of your choice.