Place the whole peppercorns in a pestle and mortar and lightly crush down, then add the sea salt and gently combine, ensuring you leave some texture. Tip on to a flat plate. Pat the steaks dry with kitchen roll and press the mixed peppercorn mixture on to both sides of the steaks, using your hands. If you have time, cover with clingfilm and leave at room temperature for at least 2 hours to allow the flavours to infuse.
To make the cauliflower puree, place the cauliflower florets in a pan with the milk and season with salt. Simmer for 10-12 minutes until completely tender, then drain and reserve the cooking liquid. Place the cooked cauliflower florets in a food processor with about six tablespoons of the cooking liquid, the cream and butter. Add a quarter teaspoon of the ground white pepper and season with salt, then whizz for a few minutes until you have a smooth puree, adding a little more of the reserved cooking liquid if necessary. This can be either used immediately or left to cool, covered with clingfilm to be reheated gently when needed.
To cook the steaks, heat the oil in a large heavy-based frying pan over a high heat. Add the steaks and cook for 2 minutes on each side, then reduce the heat, add a knob of the butter and cook the steak for 4-6 minutes, turning once, depending on how rare you like your meat. Transfer the steaks to a plate, season with salt and set aside in a warm place to rest while you make the sauce.
Pour away any excess fat from the pan and sauté the mushrooms for a few minutes until tender, then tip on to a plate and reserve. Quickly deglaze the pan with the Madeira, scraping the bottom with a wooden spoon to release any sediment and then pour in the beef stock and allow to simmer down for a couple of minutes until slightly reduced and thickened. Tip in the cooked mushrooms and allow to warm through, then whisk in the remaining butter until you have achieved a smooth glossy finish and remove from the heat. Place the rested steaks on warmed plates with the cauliflower puree and spoon over the roasted mushroom sauce to serve.