Into a slow cooker, place the lamb, carrot, celery, onion, garlic, herbs, tomato paste, worcestershire sauce and stock. Combine well and submerge the lamb under the liquid. Cook on high for 8–10 hours.
Remove the herb stalks and shred your lamb pieces with a fork to break them down a bit. Add the frozen peas and corn and cook for a further 10 minutes. Season really well.
To make the topping, boil or microwave the potatoes until tender. Mash the potatoes with the butter, warm milk and parmesan cheese and add plenty of salt and pepper.
1 kg potatoes, 50 g butter, 1/2 cup milk, 1/2 cup parmesan cheese, sea salt & cracked black pepper
Preheat the oven grill or turn the oven to 220°C conventional (200°C fan-forced). Transfer the lamb mixture to a 2.5 litre casserole dish. Top with the mashed potato. Scrap a fork across the mash to create lines in the potato. Grill for 10 minutes or bake for 20 minutes.
1 kg lamb shoulder, 1 kg potatoes
If you like, you can split the pie between two smaller casserole dishes, then freeze one of them for a later date.