Preheat the oven to 180°C.
Trim the fat from the kidneys and melt it in a large casserole over medium heat until golden-brown.
Coat the mutton in flour and brown the meat and kidneys in batches.
Layer the meat, half the onions, and bay leaves into the casserole followed by the remaining onions.
Pour in the Worcestershire sauce and beef stock and bring the mixture to a simmer.
Arrange the 5mm potato slices on top in an overlapping pattern.
Season with salt and pepper, dot with butter, cover, and bake for two hours.
Boil the heritage carrots in salted water for 3 to 5 minutes until tender.
Drain the carrots and toss them with butter and seasoning before serving.