Take a slice of grapefruit and run the juicy side along the rim of a rocks or whiskey glass. Fill a small, flat plate with coarse salt. Holding the glass upside down, dip the rim into the salt and twist clockwise, coating the rim in salt. You may have to hold the glass at an angle to fully coat it.

Place glass upright. Fill with ice. Use a measuring jigger to measure gin. Pour gin over the ice.

Pour grapefruit juice over the gin, filling the glass nearly to the top. Using a stirring spoon, give it a few stirs to mix the juice and gin.

Drop a sprig of rosemary into the glass. Sip and enjoy.