Line a half baking sheet with foil and top with an oven-safe cooling rack. Set aside.
Trim off excess fat, silver skin, and the chain muscle of your beef tenderloin. Wrap the thin tail end underneath itself to make tenderloin even in size across. Wrap tenderloin in butchers twine, in 1 to 2 inch segments, using the butchers twine to make the tenderloin as even in size as possible. (If needed, see my section above titled "Helpful Videos" for video tutorials on prepping and tying a beef tenderloin)
Generously season your tenderloin all over with salt and pepper. Place onto prepared baking sheet. Refrigerate uncovered for 24-48 hours. Doing this deeply seasons your meat and dries the outer layer for a nice brown crust when roasted.
Beef tenderloin: day of cooking
Remove tenderloin from refrigerator to bring to room temperature for 2 hours. If not, your tenderloin will take significantly longer to roast in the oven and will cook unevenly.
Preheat oven to 225 degrees Fahrenheit.
Place baking sheet with tenderloin in the oven. Roast until internal temperature reaches 5 degrees below desired doneness (temperature will continue to rise out of the oven):- Rare: 125 degrees (remove at 120 degrees)- Medium-rare: 135 degrees (remove at 130 degrees)- Medium: 145 degrees (remove at 140 degrees)- Medium-well: 150 degrees (remove at 145 degrees)Test for doneness using a meat thermometer in the center of your tenderloin. Roasting will take anywhere from 1.5 to 2.5 hours, depending on the size, shape, and thickness of your tenderloin.
Optional: Adjust oven rack so that tenderloin will be 6 inches below broiler. Preheat broiler on high. Return tenderloin to oven and broil for 30 seconds on each side, using tongs to turn your tenderloin. Remove from oven and allow to set for 10 minutes, undisturbed. (Meanwhile, prepare your mushroom sauce below).
Cut off and discard twine. Slice tenderloin into 1 inch steaks and serve with prepared sauce.
Creamy mushroom sauce
In a skillet over medium heat, melt butter. Add sliced mushrooms and cook until lightly browned, about 4-5 minutes. Add garlic and cook an additional 30 seconds.
Add beef broth and scrape the bottom of the pan to deglaze, until most of beef broth has evaporated. Add heavy cream, thyme, salt, and pepper.
Bring sauce to a simmer and reduce heat to medium-low, stirring occasionally, until sauce reduces slightly and begins to thicken, about 3-4 minutes. Add parmesan cheese and stir until melted.
Remove pan from heat and allow to set for 5 minutes to thicken slightly. This sauce is very rich and on the thin side. A little goes a long way!
Drizzle a few spoonfuls of sauce over prepared tenderloin steaks and serve immediately.