1. In a small bowl, combine orange zest, chili powder, and salt. Rub tenderloin with spice mixture. Set aside for 30 minutes.
2. Preheat oven to 450°.
3. In a large ovenproof skillet, heat oil over medium-high heat. Sear the beef on all sides until it is brown and caramelized, about 4 minutes per side (1216 minutes total). Transfer skillet to oven and roast until an instant-read thermometer registers 125° for medium-rare (1520 minutes).
4. Transfer beef to a cutting board to rest for 20 minutes, then thinly slice. Top each baguette slice with sliced beef, about 1 teaspoon green sauce, and, if using, herbs.