So I'm hosting a dinner for 4 to which one guest is bringing a (to me) nice bordeaux, a 1996 Pichone Lalande, which is a Pauillac, and which will be prefaced by an Arneis and followed by a Valpolicella. Another guest volunteered to bring meat and has called saying the meat will be a whole (trimmed) beef tenderloin.
Now I've never roasted a tenderloin (or even make filet mignons). Almost always when I make beef I buy hanger steak and when I want to splurge I buy ribeye. The fact that I'd never buy a whole tenderloin myself makes me especially excited have one brought over. But I need advice on what to do with it.
1) one guest keeps kosher, such that I can't use any dairy product or pork or any non-kosher animal product (but it's fine to use my pans, which have rendered many a strip of bacon). The tenderloin is kosher, but I don't have time to buy any further kosher meat or stock. So the sauce for the tenderloin can't include butter, cream, meat stock of any sort, etc. This feels sort of limiting.
2) I've never cooked tenderloin before, and am scared of it coming out too dry.
One suggestion has been to cover the roast in mushroom duxelles, which would add flavor and keep the meat moist. Another idea I had was to drizzle over it a reduction of balsamic vinegar.
I'd welcome adivce of any sort for cooking or saucing this tenderloin. I'm planning to serve it with a potato galette (the pommes anna from Bistro Chez L'Ami Lois as reported by Calivin Trillin) while I'll fill with sauteed ramps, plus some baby vegetables blanced and simmered in veggie stock, and a salad of mixed baby lettuces and fiddleheads. For the Arneis I'm making an appetizer of pissaladiere (slow-caramelized onion tart [sometimes a pizza but I prefer a tart] with nicoise olives and anchovies).