Place ice, tequila or vodka, grapefruit juice, lime juice, and St. Germaine in a 16-ounce jar. Crush the mint sprig with your hands to release the mint oils, then add the the jar. Shake vigorously for 15 seconds to combine.
Top the jar off with ice, then top off with soda water. If desired, slide a couple grapefruit wheels down the side of your glass to make things fahn-cy.