Pour the peppercorns into a dry cast-iron skillet and place over medium heat. Toast until fragrant and lightly browned, about 1-2 minutes.
Pour the peppercorns into a mortar and pestle and crack the peppercorns until they reach the desired consistency. I prefer mine slightly coarser than can be achieved with a peppermill.
Place the crushed peppercorns in a bowl and add the salt, stir to combine. Sprinkle the mixture on all sides of the steak, rubbing it into every nook and cranny. Place the steak on a cooling rack above a baking sheet. Place the steak in the fridge and let sit, uncovered, for 1 hour.
Pour the neutral oil into a large cast-iron skillet and place over medium-high heat. Once the skillet is smoking, add the steak. Cook the steak, flipping frequently and sealing the sides until the steak reaches an internal temperature of 120°F. Add the butter, ginger, chilis, and the light green and white parts of the scallions. Tilt the pan and baste the steak with the butter and aromatics until it reaches an internal temperature of 125°F for medium-rare. Aim higher or lower based on your steak preferences.
Transfer the steak to a small baking sheet and tent with foil. Let rest for 10-15 minutes. While the steak is resting, saute the shallots in the cast-iron skillet until translucent. Add the mushrooms and saute until browned. Deglaze the pan with the Shaoxing wine and pour in the beef stock.
Bring the mixture to a gentle simmer and stir in the creme fraiche. Add the Dijon mustard and bring to a simmer once again.
Spoon a tablespoon or two of the gravy into a small bowl. Add the cornstarch and whisk to combine. Pour the mixture into the gravy and stir until thick enough to coat the back of a spoon.
Thinly slice the remaining dark green portions of the scallions and set aside. Discard the steak aromatics and slice the steak into thick strips. Transfer 4-5 slices to a plate and cover with a spoonful of the shiitake gravy. Sprinkle with the scallions and serve immediately.