If you do not want your kidneys to taste too strong, soak them in a bowl of milk for 30 minutes, then drain.
To prevent the kidneys from curling up when cooking, soak in a bowl of hot water for 2-3 minutes, then rinse with cold water to refresh.
Mix the flour, salt and paprika together in a bowl and use to dust the prepared kidneys.
Melt the butter in a small frying pan and fry the kidneys over a low heat for 2-3 minutes on each side, or until golden-brown all over but still slightly pink inside.
Drizzle the Worcestershire sauce, vegetable stock and mustard around the kidneys and simmer until the liquid reduces slightly and the kidneys are cooked through.
Pile the kidneys onto hot buttered toast and serve immediately.