In a small bowl, combine onion, red wine vinegar, and 1/4 teaspoon salt, stirring to dissolve salt. Let stand at room temperature for 2 hours, stirring occasionally.
Meanwhile, preheat oven to 450 degrees F. Season tenderloin on all sides with salt and pepper. Place tenderloin in shallow pan and roast for 20-25 minutes, or until internal temperature reaches 145 degrees F. Remove pork from oven and let rest 3 minutes. Cut pork into thin slices. Drain onion mixture.
Arrange bottom halves of rolls on a work surface. Spread with cranberry sauce, and then top with pork and pickled onions, dividing all evenly. Add top halves of rolls and serve.