Preheat your oven to 225°F (107°C).

In a spice grinder or using a mortar and pestle, coarsely grind the black peppercorns. In a small bowl, mix the ground pepper with salt.

Pat the sirloin steak dry with paper towels. Rub the steak all over with olive oil, then coat it generously with the black pepper and salt mixture. Let it sit at room temperature for about 30 minutes.

Place the seasoned sirloin on a wire rack set over a baking sheet. Roast in the preheated oven for about 1.5 to 2 hours, or until the internal temperature reaches 125°F (52°C) for medium-rare.

While the steak is roasting, prepare the truffle and cauliflower purée. In a large pot, bring salted water to a boil. Add the chopped cauliflower and cook until tender, about 10-12 minutes. Drain and set aside.

In the same pot, combine the cooked cauliflower, heavy cream, truffle oil, and butter. Using an immersion blender or a regular blender, purée the mixture until smooth. Season with salt and pepper to taste. Keep warm.

Once the steak has reached the desired temperature, remove it from the oven. Let it rest for at least 15 minutes before slicing.

To serve, slice the steak against the grain and arrange it on plates. Spoon the silky truffle and cauliflower purée alongside. Garnish with freshly chopped chives and a sprinkle of cracked black pepper.