In a small bowl, combine onion, olive oil, balsamic, thyme, rosemary, lemon zest, garlic, salt, and pepper. Rub over tenderloins. Place on a rack in a shallow roasting pan.
Bake, uncovered, at 400℉ until the meat is 160℉, about 25-35 minutes. Cover with foil; let rest for 10 minutes before slicing. Enjoy!