Season the tenderloin all over with salt. Refrigerate 4 hours or up to 24 hours. Thirty minutes before roasting remove beef from refrigerator
Preheat oven to 400 F. Combine mushrooms, rosemary and peppercorns in spice grinder. Grind to a coarse powder. Rub beef with olive oil. Coat all over with porcini rosemary rub. (Note: Make sure your mushrooms are completely dry and brittle, or they won’t easily grind into powder.)
Heat 1 tablespoon olive oil in a wide skillet over medium-high heat until shimmering. Add beef and brown on all sides, about 8 minutes. Transfer to a roasting pan. Roast in oven until thermometer inserted in thickest part reads 120- 125 F., about 30 minutes, for medium rare. Remove from oven and transfer to a cutting board. Tent with foil and let stand for 15 minutes.
Carve beef in 1/4 inch slices. Serve with Port Wine Sauce.
For the port wine sauce:
Strain the porcini water through a paper towel into a small bowl. Reserve strained liquid. Coarsely chop porcini. Using the same skillet from browning the meat, add 1 tablespoon butter, shallots and chopped porcini. Sauté over medium heat until shallots are translucent, about 2 minutes.
Add port wine, scraping up any brown bits in the pan. Add red wine, mushroom stock and rosemary. Bring to a boil and cook uncovered until sauce is reduced by about half to approximately 1 1/2 cups. Add salt and taste for seasoning. Strain through a fine-meshed seive into a small saucepan, pressing firmly on solids. Discard solids.
Heat sauce over medium heat. Whisk in 2 tablespoons butter, 1 tablespoon at a time. Keep warm until serving.