You can make something truly celebratory and festive with this recipe without too much trouble and expense, although you do have to be willing to slice up lots of fresh mushrooms.
Rub tenderloin all over with salt, let sit at room temp while you prep and cook the mushrooms, or at least an hour.
Heat oven to 425.
Rinse mushrooms and wipe clean, slice fairly thickly, then saute in olive oil over medium heat. Make sure you get the mushrooms cooked down so that there's little to no liquid in the pan. You'll probably need to do this in a couple of batches, depending on the size of your pan. Sprinkle generously with pepper and add the garlic and flour, stirring around to distribute evenly. Scrape the mixture into the bottom of the roasting pan you're going to use. Use some of the red wine to loosen any remaining flour/mushroom bits from the pan and scrape that in, too, then add the remaining wine. to the roasting pan. Put a rack over the mushrooms and place your tenderloin on that. You can also just put the meat on top of the mushrooms if you don't have a rack, but I think the meat cooks better if it's a little elevated and the bottom isn't poaching in liquid.
I think the times given for roasting a piece of meat this size are too short. I don't want raw meat! I was very pleased with an internal temp of 135, which took about an hour. Be sure you're checking the temp in the very middle of the roast. The meat was good and pink, and very tender. You don't want to go beyond that temp, so start checking at around 45 minutes. You can always cook it longer, but you can't cook it shorter.
Take the meat out of the roasting pan and put it on a cutting board, covering it with foil. I like to put it on the back of the stove if I can; my big cutting boards are made of plastic right now, though, so that isn't a good idea. Tuck it into the corner of your counter next to the stove and it should stay warm.
Make the gravy/sauce: Scrape down the sides of the roasting pan, getting all the mushrooms down from the sides, and add two cups of water. Put the pan over your biggest burner and bring the mixture to a boil, being sure to scrape up every last bit from the bottom. Then taste. You'll probably want to add some beef base just to give it a little extra flavor, but you won't need much. The mixture should be thick enough to be considered a gravy--remember, you added flour at the outset. Since a tenderloin is quite lean, you won't have too much fat in this gravy, so you won't have to do the horrible fat-skimming-at-the-last-minute routine.
Pour the mushroom mixture into a bowl or gravy boat, slice the meat and put it on a platter or in a chafing dish pan, and serve.
*I find Costco's meat to be excellent and a good price. Their beef tenderloins are more like 7-9 pounds, so I cut off part and froze it. Five pounds fit perfectly into my big roaster, and I made sure to include the thickest end. Ten people pretty well demolished this amount of meat, so if you're serving more people you could cut your roast in half and put the pieces side-by-side on your rack, turning them around the other way midway through, although that would be quite an operation. Or you could cook the extra meat in a separate pan, maybe one of those disposable ones, putting it in the oven 15 minutes after the original pan goes in.
**I say to use three pounds of mushrooms because I just bought two of the 24-ounce packages at Costco. In reality, you almost can't have too many mushrooms. If I had realized how much people would love the mushrooms/sauce I'd have bought a third package. Yes, prep takes awhile, but they're so good, and the rest of this recipe is so simple, that it's totally worth it.
***I'm including the "minced" option because not everyone has a garlic press, but honestly, folks! Just get one and use it! I got very, very tickled by this Pinch of Yum post which goes into great detail about how to peel garlic and implies that you'll then mince it, but at the end of the post they list a garlic press as one of their favorite tools, and if you use a press you don't have to peel the garlic. To quote the comment I posted on that site (and never got a reply): "Just use your paring knife to cut off the hard top of the clove and pull down/off a strip of the peel, put the clove peeled side down into the press, and . . . press. You do have to clean out the press after each clove or two, but that’s pretty easy. If you want a big batch of roasted garlic, then of course you don’t peel it until afterwards, when you just squish out the roasted garlic between your fingers."