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Tenderloin is infamous for its ultra-tender texture, extreme leanness, and very mild flavor. While the more robust flavor of bison adds a bit to the flavor department, even a bison tenderloin can benefit from a good spice rub to amp it up. The mildly smoky, raisin-like intensity of ancho chili powder will do nicely.
I follow Josh Bousel's advice for building the rub, starting with an even mix of kosher salt and brown sugar, then adding my main chili, and smaller amounts of other flavors: cumin, coriander, Mexican oregano, and black pepper.
Tenderloins are soft and tend to get a little flat when cooking, so it's best to tie them off at 1-inch intervals using butcher's twine. You can go with the fancy-pants butcher's knots I've used here (I really should put together a guide to knots some time), or just cut off multiple pieces of string and go with simply granny knots. The bison won't know the difference.
Letting the bison rest for at least 45 minutes after applying the rub will ensure proper adhesion and reabsorption of any of the juices that might have gotten drawn out by the salt. While it rests, you can make the cilantro sauce, which is essentially a modified chimichurri, made with fresh cilantro, red pepper, garlic, vinegar, and olive oil.
Tenderloin is lean, so it cooks relatively fast, which means that slow-roasting will deliver the best results if an even medium-rare is what you're after. Normally I'd suggest the reverse-sear method—starting it in a low and slow oven, finishing it in a ripping hot skillet—but in this case, I've found that with rubbed meat, the rub stays on better and you get more even browning (as opposed to blackening) if you sear first then finish.
Because the rub is high in sugar, searing takes less than half the time a bare piece of meat would take, leading to very little overcooking in the process. I sear in a moderately hot cast iron pan.
After a short stay in a 275°F (just long enough to come up to 125°F on an instant-read thermometer), I take the roast out, let it rest a few moments, then carve it up.
The flavors are powerful, the meat is tender, the colors are festive, and it doesn't take all that long to cook. What more could you want from a holiday roast?