Combine olive oil, vinegar and rosemary. Place pears in marinade and refrigerate for two hours. Drain well.
Using a mixer, whip cheese and milk in a small bowl on low speed until a whipped-cream texture is obtained. Add chives and season to taste.
Stuff each pear half with cheese mixture. Sprinkle with walnuts and drizzle with balsamic vinegar. Top with chives.