Put the strawberries in a large bowl and sprinkle over the sugar, then carefully turn the berries so they are completely coated. Leave for 3 hours so the fruit macerates and gives off some juice.
Add the brandy, if using, and leave for a few minutes before putting the mixture in a very large jug. Carefully top up with the prosecco or cava, making sure it doesn't fizz over the top. Decorate with mint and fresh strawberry slices.
For the non-alcoholic version, add 1·5 litre (2½pt) soda water plus 750ml (1¼pt) white grape juice to the strawberries in the same way.