Heat a little butter in a non-stick frying pan or wok and brown the beef in batches. Remove and keep aside. Similarly brown the kidneys, remove and keep with the beef.
Fry the onion, garlic and carrot in the pan with a little more butter until the onion is golden. Sprinkle over the flour, stir and allow to cook for a few minutes then shake over the Worcestershire sauce. Pour in the stock, and bring to the boil. Stir in the beef and kidney and the bay leaf, then turn off the heat.
Preheat the oven to around 180 deg C.
Place all the cooked ingredients into a casserole, cover, and cook in the oven for around 30 minutes.
Remove from the oven and arrange the sliced potatoes on top of the meat, a little like roofing tiles. Brush a little more butter over the potatoes and season with salt and freshly ground black pepper. Re-cover and return to the oven for about 1 hour or until the potatoes are cooked.
Remove the lid, brush the potatoes with a little more butter and finish under the grill until the potatoes are brown and crispy.