Preheat oven to 425° F. Coarsely chop onion. Pat pork dry and season with salt and pepper. In an ovenproof large heavy skillet heat oil over moderately high heat until hot but not smoking and brown pork. Transfer pork to a plate and in oil remaining in skillet cook onion over moderate heat, stirring, until golden. Add Port, orange juice, and vinegar and simmer, stirring, 2 minutes. Add fresh or frozen cherries, caraway seeds, pork, and any juices that have accumulated on plate and bring to a boil. Transfer skillet to middle of oven and roast pork 30 minutes, or until a meat thermometer registers 155° F. Transfer pork to a cutting board and let stand, loosely covered with foil, 10 minutes. While pork is standing, simmer sauce, stirring, until slightly thickened and reduced to about 1 1/2 cups. Cut pork into 1/2-inch-thick slices and spoon sauce over it.