For the mushrooms: In a saucepan over medium-high heat, bring the oil to a light simmer and reduce to medium heat. Then add the mushrooms and cook until the exterior of the mushrooms softens, about 4 minutes. Remove and hold on a plate until serving.
For the steak: Preheat the oven to 400 degrees F. Warm the prepared potatoes and hold warm. Heat a thick-bottomed pan until the verge of smoking over medium-high heat. Then sprinkle the steaks with salt and pepper on both sides. Apply the steak to the heated pan. Sear on the first side for 4 minutes. Reduce the heat to medium, and then flip and repeat the process on the second side. This will result in a medium-rare steak. Finish the oven for higher temperatures.
Once the steak is cooked, remove from the pan to rest, and then add the peppercorns to the pan and warm for 20 seconds. Then add the demi-glace and garlic, warming for 3 minutes. Once warmed, remove from the heat and stir in the butter until smooth.
To serve, place a quarter of the mashed potatoes in the center of a plate, then place the cooked steak and drizzle with 1/4 cup sauce. Finish with a quarter of the mushrooms and 5 pieces of poached asparagus and serve.