Put the raspberries, apple juice, and vinegar in a blender and process on high speed until the raspberries are pulverized. Pour the mixture through a fine-mesh strainer to remove the raspberry seeds. Rinse the blender container to remove any seeds, then return the raspberry mixture to the container. Add the walnuts and oregano and process on high speed until smooth. Stored in a sealed container in the refrigerator, Raspberry-Walnut Dressing will keep for 2 days.