Line a rimmed baking sheet pan with foil and set a wire rack inside the pan. Spray the wire rack with nonstick cooking spray. Set aside.
Tie the tenderloin with cooking twine at 1-2 inch intervals, being sure to tuck and tie the tail end underneath by 3-5 inches to create a more even shape. Generously season the beef with salt and pepper.
6 pounds beef tenderloin, Kosher salt, Freshly ground black pepper
Center the beef on the rack and refrigerate for up to 24 hours.
30 minutes before cooking, preheat the oven to 250°F. Remove the beef from the refrigerator. Set aside while the oven heats.
Place the tenderloin on the baking sheet on the center rack of the oven and cook for 1 hour.
Remove from the oven and pour half of the melted butter over the tenderloin.
½ cup unsalted butter
Return the beef to the oven and cook until the middle of the tenderloin registers 130-135°F (medium-rare), 140-145°F (medium), or 150-155 °F (medium well). The cook time can vary anywhere between 1 ½ - 2 hours.
While the tenderloin cooks prepare the mushroom sauce and Panko gremolata.
Remove the beef from the oven and set the oven to broil.
Brush the remaining melted butter over the tenderloin and return it to the oven to brown. Watch the beef closely so it doesn’t burn and cook it just long enough to add a little color.
Remove the beef from the oven, tent it loosely with foil and allow it to rest 10-15 minutes before slicing.
Remove the twine, and cut into ¾-inch slices, and serve with mushroom sauce (below) and Panko gremolata (below).