Combine the white wine vinegar, sugar, coriander seeds, cumin seeds, bay leaf, salt, and onion in a small saucepan and bring to a boil over medium heat. Boil gently for a minute or two, stirring occasionally, until the sugar is completely dissolved.
Pour the brine over the raisins. Refrigerate overnight before using. The pickled raisins will keep for 2 weeks or more, tightly covered in the refrigerator.