In a tall glass, add 10 fresh mint leaves and 1 large lime, cut into wedges.
Add 2 tsp caster sugar and gently muddle the mint and lime with a muddler or the end of a wooden spoon until the essential oils are released.
Pour in 50 ml of good-quality white rum and stir gently to dissolve the sugar.
Fill the glass to the top with crushed ice.
Top up the glass with soda water, stirring gently from the bottom up to combine without losing the fizz.
Garnish with a sprig of fresh mint and a slice of lime before serving.