Preheat the oven to 170°C.
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.
Spray a large casserole dish with low-calorie cooking spray and add the potato, carrot, onion, pepper, tomatoes and garlic and season well.
Place the lamb on top of the vegetables.
Pour in the lemon juice and water. Cover with a lid and cook for 3 hours. Check halfway through and add a little extra water if the vegetables look like they are drying out.
Turn up the oven to 200°C.
Remove the lamb from the oven and shred with two forks.
Return the dish to the oven for 20 minutes to crisp up the lamb around the edges. Serve with an accompaniment of your choice.