Heat your oven to 450°. Place the tenderloin in a baking pan, and rub it with the olive oil. Season with salt and pepper.
Bake the tenderloin uncovered for 5 minutes for each pound, or 12 1/2 minutes for a 2 1/2 pound tenderloin. Remove the tenderloin from the oven and allow to rest for 10 minutes.
Meanwhile, to make the garnish, heat the olive oil in an 8” skillet. Add the onion and sauté for 2 minutes, until it is slightly browned.
Add the nanches and jalapeños, and sauté for another 3-4 minutes. Salt and pepper to taste. Remove from the heat, and serve alongside the beef tenderloin.