Preheat your oven to 180°C (355°F). In a 2-litre shallow casserole dish or a non-stick frying pan, heat a generous amount of light oil over medium-high. Season the diced lamb with salt and black pepper, add to the hot pan, and brown on all sides. Once browned, use a slotted spoon to remove the lamb and set aside.

While the lamb cooks, peel and finely chop the onion. Add it to the pan after removing the lamb. Sprinkle with a few generous grinds of salt, reduce the heat to medium, and cook gently for about 10 minutes until the onion is soft and turning golden, adding a bit more oil or a splash of stock if it starts to stick or the pan looks dry.

Sprinkle in the flour and stir, letting it cook for 1 minute. Pour in half of the stock, scraping up any browned bits from the base of the pan. Add the remaining stock, thyme sprigs, bay leaf, a generous shake of Worcestershire Sauce, carrots (peeled and chopped into chunks), and the browned lamb. Season again, bring everything to a boil, cover with a lid (or transfer to a baking dish and seal tightly with foil), and bake in the oven for 30 minutes.

Meanwhile, peel the potatoes and slice them thinly. Set the slices aside and melt the butter if using.

After 30 minutes, remove the lid from the casserole and take out the thyme sprigs and bay leaf. Give everything a stir, then neatly layer the potato slices on top in a spiral. Brush with the melted butter or spritz with olive oil spray. Sprinkle the potatoes with a little extra salt and plenty of black pepper. Return the dish to the oven, uncovered, for another 40 minutes, until the potatoes are crisp and golden at the edges.