Crush your whole black peppercorns roughly using a pestle and mortar. You want distinct, chunky bits, not a fine powder. Coat your steak lightly in a tiny bit of oil, then press the crushed pepper firmly into one side of the meat so it forms a solid layer.
Preheat your oven to 220°C (425°F). Heat a trusty cast-iron skillet over medium-high heat with a splash of oil. Right before the steak hits the pan, drop in a knob of butter. Sear the steak for about 2 minutes on each side to build a gorgeous, dark crust.
Transfer the steak to a tray and pop it into the preheated oven for 5 minutes. The gentle ambient heat of the oven ensures the thick steak cooks through evenly.
Remove the steak from the oven and transfer it to a board or plate. Let it rest for 5 minutes. Do not skip this! Resting allows the muscle fibres to relax and reabsorb the juices; if you cut it too early, those delicious juices will flood the board.
While the steak rests, turn the stove heat down to medium-low. Pour the Cognac straight into the pan juices.
Tony's Pro Tip: Don't intentionally set the pan on fire. Bourdain explicitly noted that massive pillars of flame are just annoying and dangerous in a home kitchen. Instead, let the liquid bubble gently while using a wooden spoon to aggressively scrape up all the fond—the caramelized, crusty beef bits stuck to the bottom of the pan.
Once the Cognac has reduced down to a thick, maple-syrup-like consistency , stir in the demi-glace. Let it melt into the sauce. Next, add a tiny splash of cream.
To finish the sauce in classical French style, drop in a cold knob of butter and whisk it smoothly into the pan (monter au beurre). Taste the sauce, add a final hit of salt to transform the flavors, and pour it generously over your rested steak.
