Heat 30 grams lard, dripping or butter in a frying pan and brown 900 grams lamb shoulder, in batches. Remove from the pan.
Core and finely chop 3 lamb's kidneys and brown all over. Set aside.
Add 4 brown onions, sliced, to the pan (you might need to add a little more fat) and cook for a few minutes until they begin to soften.
Add 4 carrots, peeled and sliced, and cook for another few minutes.
Sprinkle over 25 grams plain flour, stir to combine and cook for another minute.
Return the lamb and kidneys to the pan.
Add 500 ml lamb stock and 1 tbsp Worcestershire sauce and bring to a simmer.
Add 1 tbsp fresh thyme leaves and 2 bay leaves, pop the lid on and allow to simmer while you prepare the potatoes.
Wash and thinly slice 1.5 kg rooster potatoes into 2-3mm thickness. Leave them unpeeled if you wish.
Grease a large, lidded casserole dish and arrange half of the potatoes on the bottom and sides. Season well with Shetland sea salt and freshly ground black pepper.
Spoon in the lamb mixture.
Arrange another layer of thinly sliced rooster potatoes over the top of the lamb mixture, making sure the potato slices overlap slightly in a thatched effect.
Brush with a little melted lard, dripping or butter and season well.
Bake in the centre of the oven for one hour, with the lid on.
After one hour, turn the heat up to 200 °C/ 180 °C fan, remove the lid and bake for a further 30 minutes to allow the top layer of potatoes to crisp up.