Heat the olive oil and butter in a frying pan until foaming, then fry the bacon for 2-3 minutes, or until golden-brown. Add the baby onions and cook for 1-2 minutes, or until softened.
Add the kidneys and cook for 1-2 minutes, or until golden-brown all over. Add the Madeira and sherry to the pan and carefully tip the pan towards the flame of the hob, so that the alcohol catches alight (NB: keep clothing and fabric well clear of the flame). When the flame has gone out, cook until the alcohol has reduced in volume by half.
Add the beef stock to the pan and simmer for 3-4 minutes, or until the kidneys are cooked through. Stir in the cream and cook for 1-2 minutes to warm through, then season, to taste, with salt and freshly ground black pepper. Stir in the parsley. Set aside and keep warm.
Heat a griddle pan until hot, then add the slices of sourdough and toast for 1-2 minutes on both sides, or until crisp and golden-brown.
Whisk together the mustard, vinegar and extra virgin olive oil in a bowl until well combined, then season with salt and freshly ground black pepper. Drizzle the dressing over the salad leaves and mix well to coat.
To serve, place two pieces of toast in the centre of each of four serving plates. Spoon the kidneys and sauce over the top. Divide the salad among the plates and arrange alongside the kidneys and toast.