Start by making dill pollen-infused sherry.
With a sous vide machine: Put the dill pollen into a vacuum bag along with the fino sherry. Seal the bag and cook in a preheated water bath for 45mins at 50C. Allow to cool, and then filter through a coffee filter paper and reserve the infused sherry.
Without a sous vide: Put the pollen and sherry in a large clean jar or bottle with a well-fitting lid or stopper. Leave to infuse for 24hrs. Agitate the mix a few times during this period to encourage the flavours to be released.
Pour 35ml of your dill pollen-infused sherry into a champagne flute and add the elderflower cordial and prosecco. Stir gently to combine, then taste. Add a little sugar syrup if it needs extra sweetness, then stir again before serving.