Heat a large frying pan and melt the butter.
Add shallots and cook for a few minutes, followed by garlic for one minute.
Increase heat and fry kidneys until golden-brown, then add mushrooms for two minutes.
Deglaze the pan with brandy, scraping up any charred bits.
Add beef stock, bring to a boil, and reduce by half.
Stir in cream, mustard, Worcestershire sauce, and cayenne, then simmer for 3 minutes to thicken.
Drizzle oil over sourdough and toast in a griddle pan for one minute per side.
Whisk olive oil, walnut oil, mustard, and vinegars together in a small bowl.
Stir parsley into the kidneys and serve on top of the toasted bread.
Dress the lettuce leaves and serve alongside the kidneys.