Preparation

Make the sauce

• Soak the chiles in hot water to soften, about 10 minutes. Drain and transfer to a blender. Add the orange juice, vinegar, garlic, oil, cumin, cayenne, and 1 tsp. salt, and blend until smooth, about 2 minutes. (If not using right away, cover with plastic wrap and refrigerate for up to 3 days; the sauce can also be frozen.)

Make the tacos

• Position a rack in the upper third of the oven and heat the broiler. Line a large rimmed baking sheet with foil.
• Make a lengthwise slit to butterfly the tenderloin, taking care not to cut all the way through to the opposite side. Open the tenderloin, cover with a sheet of plastic wrap, and gently flatten the meat with a meat pounder to an even thickness. Transfer to the prepared baking sheet. Pour half of the sauce over the tenderloin, and rub to coat the meat evenly. Put the rest of the sauce in a small serving bowl. Broil until charred in places, 3 to 5 minutes. flip and broil until cooked through (140°F). Transfer to a cutting board, tent loosely with foil, and let rest for 5 minutes. Slice thinly and serve in warm tortillas topped with more sauce, avocado, onion, cheese, and cilantro, if you like.