1/2 ounce dried porcini mushrooms, reconstituted in 3/4 cup water, liquid reserved
3 tablespoons unsalted butter, divided
1 medium shallot, finely chopped
1 cup port wine
1 cup heavy bodied red wine
2 rosemary sprigs
1/2 teaspoon salt, or to taste
For the Beef Tenderloin:
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 willingly grind into powder. If they're at all pliant, you can dry them out in the oven until they break easily.) 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 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 an un-bleached 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.
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