How to Make It
Start the shrub: Grate zest from grapefruits with a Microplane to equal 2 ½ tablespoons; place in a large bowl. Cut away peel and pith of grapefruits; discard. Chop flesh into 1-inch pieces. (you should have about 2 cups.)
Add sugar to zest in bowl and rub with fingertips to release natural oils. Stir in chopped grapefruit and ginger. Cover with plastic wrap and let sit at room temperature, stirring occasionally, for at least 8 hours and up to overnight.
Stir vinegar into grapefruit mixture. Cover and refrigerate for 2 days. Strain mixture through a fine-mesh sieve into a 1-quart lidded jar. (You can store shrub in refrigerator for up to 3 months.)
To serve: Pour 3 tablespoons shrub into an ice-filled glass; top up with seltzer and garnish with a grapefruit slice.