01 - Zest one grapefruit using a fine grater and set the zest aside for later use.
02 - Juice all grapefruits to yield about 1.5 cups (360 ml). Strain thoroughly to remove pulp and seeds.
03 - Combine sugar and water in a small saucepan. Bring to a simmer over medium heat, stirring constantly until sugar dissolves completely. Remove from heat and cool to room temperature.
04 - In a large bowl, mix grapefruit juice, zest, lemon juice, salt, and cooled simple syrup until well incorporated.
05 - Refrigerate the mixture for at least 1 hour until thoroughly chilled.
06 - Pour chilled mixture into an ice cream maker. Churn according to manufacturer's instructions for 20–25 minutes until thick and slushy.
07 - Transfer sorbet to a freezer-safe container. Cover and freeze for at least 4 hours or until firm.
08 - Let sit at room temperature for 5–10 minutes before scooping. Garnish with fresh mint leaves if desired.