Preheat oven to 400°F. Melt butter in a medium saucepan over medium heat. Add onions and cook for 5 minutes or until soft. Stir in dates, breadcrumbs, nuts and herbs.
Using a sharp, non-serrated knife, cut a pocket into the thickest part of each chicken breast, being careful not to cut al the way through. Leave about 1/2-inch on 3 sides. Fill pockets with date mixture.
Season with salt and pepper and brush both sides with olive oil. Place in a shallow baking dish and add wine to dish.
Bake for 30 to 35 minutes or until chicken reaches 165°F internally.