Preheat the oven to 180°C, fan 160°C, gas 4.
Heat half the oil in a large frying pan and brown the lamb in batches over a high heat. Remove and place in a large shallow casserole dish. Add the kidneys to the pan, quickly brown them, then add to the lamb.
Heat the remaining oil in the pan, then fry the onions over a low heat for 10 minutes, until golden brown. Stir in the flour, then add the stock and Worcestershire sauce, and bring to the boil, stirring. Season to taste with salt and freshly ground black pepper. Add the bay leaf and thyme, then pour into the casserole dish over the lamb.
Arrange the potato slices on top of the meat and brush with the melted butter. Cover with a tight-fitting lid and cook in the oven for 1 1⁄2 hours. Remove the lid and cook for a further 35 minutes, until the potatoes are golden and crisp. Great with steamed green vegetables.