You'll need:
1 grapefruit wedge (or small amount of extra juice)
Kosher salt, for the rim
1 2-in rosemary sprig, plus one for garnish
1/2 tsp sugar
2 oz pink grapefruit juice
1 1/2 oz gin
Moisten the outer rim of a martini glass with juice or the grapefruit wedge, and carefully coat with salt. Place the glass in the freezer, or fill the glass with ice to chill. Muddle the rosemary and sugar in a cocktail shaker (reserving the extra piece for garnish). Add grapefruit juice, gin, and ice and shake to combine. Empty the cocktail glass, or remove it from the freezer. Strain the cocktail into the prepared chilled glass and garnish with the reserved sprig of rosemary. Enjoy!