Mix the raspberries, sugar and water in a small saucepan over medium heat. Stir until all the sugar is dissolved and the syrup is bubbly. Remove from heat and allow to cool slightly.
Put the syrup through a sieve to remove the solids. Chill to room temperature. You can keep the syrup in the refrigerator for up to 3 days.
To make two cocktails, pour the raspberry syrup into a cocktail shaker. Add the gin and lemon juice. Shake to combine.
Fill two tumblers with ice. Pour half the contents of the shaker into each glass. Top with tonic/soda water, a few raspberries and a sprig of mint for garnish. Serve.
Notes
This cocktail can be made with many different types of berries and fruits, like blackberries, blueberries, peaches, pears, and cranberries.
You can double or triple the recipe for the raspberry syrup if you want to make more than 2 cocktails.