In a blender, purée grapefruit, lemon, Bourbon Whiskey, grapefruit soda, agave, salt and Angostura bitters until smooth. Add ice and purée until smooth. Pour into a shallow casserole dish and freeze until slushy, about 2 hours.
Divide among glasses and garnish with mint and grapefruit, adding more grapefruit soda for a loose sipping slushie.
Notes
Tip: Reduce the work of squeezing and straining citrus by peeling, chopping and throwing it all in a blender. And don’t forgo the salt – it enhances the grapefruit flavour.