Preheat the oven to 220 to 230 degrees F (105 to 110 degrees C).
Seasons the tenderloin with 2 teaspoons salt and 1 teaspoon pepper, and let it cure.
Meanwhile, prepare the miso butter by mixing the butter and miso paste together in a small mixing bowl using a food processor.
Once the miso butter is ready, use half of the quantity prepared to coat the tenderloin roast.
Set up the roasting pan, and combine the red onions and cremini mushrooms at the bottom, then dash the ½ teaspoon of salt left along with the olive oil over the mix.
Form a longitudinal mount with the red onions and mushrooms that runs along the length of the tenderloin exactly at the center.
Lay the tenderloin over the rack of the oven pan, exactly above the red onions and mushrooms.
Start slow roasting the beef tenderloin until it reaches an internal temperature of 105 degrees F (40.5 degrees C). This should take about 1.5 to 2 hours.
Once it reaches the 105 degrees F point, remove the roast from the oven, coat with the other half of the miso butter, and tent the roast with aluminum foil. Meanwhile preheat the oven to about 500 degrees F (260 degrees C).
Once ready, return the beef tenderloin back to the oven and roast it until it reaches an internal temperature of 125 degrees F (about 50 degrees C) for medium-rare, or adjust accordingly. This should take about 10 to 15 minutes.
When the beef reaches the ideal level of doneness for you, take it out of the oven and let it rest for 15 minutes before slicing the beef and serving it with the roasted cremini mushrooms and red onions.
