Preheat your oven to 475 degrees F.
Heat a large oven-safe skillet or cast iron skillet over medium-high heat. Season the beef tenderloin heavily with salt and black pepper. Once the pan is hot, carefully add the beef to the pan. Sear the beef on all sides until brown and slightly caramelized. Remove the beef from the pan and lower the heat to medium. Add more oil if the pan has dried out, and then add the mushrooms. Cook for about 5-6 minutes or until the mushrooms are just tender. Season with salt and black pepper. Add the beer and bring to a low boil. Reduce the heat to a simmer and continue simmering until most of the beer has evaporated. Add the meat back to the pan and brush it with olive oil.
Place the beef tenderloin in the preheated oven and roast for 45-50 minutes or until a thermometer registers 120-125 degrees F for medium rare/rare. Remove from heat and transfer the beef to a large cutting board to rest. Cover it with a foil tent. Once the beef has rested for at least 15 minutes. Slice it as desired.
Serve the beef tenderloin with beer horseradish sauce and pilsner mushrooms. Enjoy!