Start this recipe the day before you wish to serve it.

Zest and juice the grapefruit, measuring the juice you get in a cup measure. Pour the juice and the zest into a storage container. Measure an equal quantity of sugar to the grapefruit and tip into the grapefruit juice. Stir to dissolve the sugar, then place in the fridge to infuse overnight.

When ready to serve, place the grapefruit syrup, tequila and lime juice in a cocktail shaker and shake to combine well. Run a cut grapefruit around the rim of the glass and dip into salt. Pour over ice in a tall glass, top with soda water and serve.