Start by washing the lime, cucumber, and mint leaves. Thinly slice the cucumber and one lime; the other lime will be juiced.
In a sturdy glass, combine the mint leaves, brown sugar (or your chosen sweetener), and lime juice. Gently muddle these together with a muddler or the back of a spoon until the mint leaves are slightly bruised and the aroma fills the air.
Add the sliced cucumber to the mix. Give it a gentle muddle to release its fresh, crisp essence into the concoction.
Fill the glass with crushed ice. The more, the merrier – it keeps the mocktail refreshingly cool.
Pour the sparkling water over the ice. Use a spoon to stir everything gently, combining the flavors while keeping the fizz alive.
Add a few mint leaves and a lime slice or two for garnish. If you're feeling extra festive, add a green straw or a decorative stick.