Preheat oven to 160C. Trim lamb of excess fat and cut into 5cm pieces.
Heat a large casserole over med/ high heat. Add oil and brown lamb in batches. Set aside.
Add onion, carrot and celery to the casserole and sauté until softened.
Add garlic and rosemary and sauté for a further 2 minutes.
Return the lamb to the pan, add Worcestershire and stock, season with salt and pepper, cover with a tight fitting lid and bake until the lamb is tender, about 2 hours.
Increase oven to 180C. Place the lamb into a bowl and place the casserole over medium heat. Place the flour into a small bowl, add a ladleful of the braising liquid and whisk until smooth. Add the flour mixture to the braising liquid and simmer until thickened.
Shred the lamb, return to the pan and mix through. Place mixture into a medium baking dish.
Slice the potatoes 0.5 cm thick and coat in the melted butter. Place over the lamb mixture in concentric circles.
Bake in the oven for 45 minutes or until the potatoes are tender and golden.
Meanwhile, mix the ingredients for the gremolata in a small bowl.
Serve hot pot sprinkled with gremolata, offering extra on the side.