Rinse celery and chop finely. Trim scallions, rinse and chop very finely. Heat the butter in a small saucepan. Add celery and scallions and sauté briefly. Sprinkle flour over the celery and scallions. Pour in milk and bring to a boil. Stir in cream cheese. Season with pepper. Keep sauce warm.
Cut pork into about 2 cm (approximately 3/4-inch) thick slices. Season with salt and pepper. Heat oil in a non-stick skillet. Add pork medallions and sear on both sides, then cook over moderate heat for 5 minutes. Thread medallions on wooden skewers and serve with the celery and cheese sauce.