Using two stemless wine glasses, pour 1.5 oz of vodka into each.
Using a glass juicer (there's a link to my favourite one on my Shop page!), or using your hands, squeeze juice from each grapefruit then pour evenly into each glass.
Add 1 tsp of elderflower cordial into each glass, along with a handful of ice cubes.
Top with sparkling water and garnish with a sprig of thyme.