Rub tenderloin with salt and pepper, half of the rosemary and half of the garlic. Heat 1 T. of the oil in a skillet. Add pork and braise on each side until golden brown on all sides, about 10 minutes. Remove from skillet and put in a greased jelly roll pan.
Add vinegar to skillet, scraping bottom to remove brown bits. Add remaining oil, rosemary, and garlic. Add potatoes and onions; toss to coat.
Spoon potatoes and onions around pork; drizzle vinegar mixture over pork.
Roast for 20-25 minutes or until pork reaches 140*. Let stand 5 minutes before serving.