Preheat the oven to 180°C (350°F), Gas Mark 4.
Melt 20g of the butter in a large pan, add your onions and sweat them down over a low heat for 15–20 minutes, until a very pale golden colour. Season with salt and a pinch of pepper. Set aside.
Next, put all your diced lamb into a large mixing bowl, dust it with flour and season with the sugar and one teaspoon each of salt and pepper. Then get your hands in there and give it a good mix, making sure all the meat is coated with the flour and seasonings. Set aside.
Now, peel your potatoes and then slice them quite thin, about 2–3mm thick. I use a mandoline for this, but you can also use a sharp knife. Put the potato slices into a bowl.
Melt another 15g of the butter, pour over the potato slices and season with a teaspoon each of salt and pepper, then once again mix with your hands to make sure all the slices are coated. Be careful not to break them, as you need them intact for layering.
Now you are ready to start layering. Use the remaining 15g of butter to grease a high-sided casserole dish and arrange a layer of potato slices over the base and up the sides – you are going to encase your lamb inside a layer of potato. Put your floured and sugar-seasoned meat into the base, add half your onions and mix together well. Then layer the rest of the onions on top.
Place the lamb cutlets around the side of your dish, pushing the flesh into the onions, with the bones poking out of the top. Sprinkle some thyme leaves on the onions before arranging the remaining potato slices on top, overlapping them like fish scales.
Pour in the stock, then brush the top with a little more melted butter. Add a final crack of pepper and salt, and a sprinkling of thyme. Bake for about two-and-a-half hours. I brush with a little more butter halfway through.