Make the raspberry syrup. Place the raspberries and caster sugar in a medium saucepan. Gently muddle the berries with the back of a fork until they begin to release their juice — you’ll see a deep crimson stain spreading through the sugar. Add the lemon zest and the cold water, then set the pan over a medium heat. Stir occasionally until the sugar has fully dissolved and the mixture is simmering gently. You’ll smell the warm, jammy fragrance of the berries as they soften.

Simmer and steep. Once it reaches a gentle simmer, reduce the heat to low and let it bubble away for 8–10 minutes. The liquid will deepen into a rich ruby colour, and the raspberries will have broken down completely, looking soft and slightly translucent. Remove the pan from the heat.

Strain the syrup. Set a fine-mesh sieve over a large heatproof bowl or jug. Pour the raspberry mixture through the sieve, using the back of a spoon to press every last drop of syrup from the solids. Discard the seeds and pulp. You should be left with a smooth, glossy, jewel-toned syrup. Let it cool to room temperature — you’ll notice it thickens slightly as it cools.

Add the lemon juice. Once the syrup is cool, stir in the freshly squeezed lemon juice. Taste it at this stage. It should be tart and sweet in equal measure, with a bright, zesty finish. If it needs more tang, add a squeeze more lemon. If it’s too sharp, stir in a teaspoon more sugar dissolved in a splash of warm water.

Chill the concentrate. Transfer the lemon-raspberry concentrate to a clean bottle or jar and refrigerate for at least 1 hour, or until thoroughly chilled. The concentrate should feel cold to the touch, and the colour will have settled into a deep, almost garnet-like shade.

Assemble the drinks. Fill tall glasses with ice cubes. For each serving, pour about 60–80ml of the chilled concentrate into the glass (adjust to your taste). Top up with sparkling water, pouring slowly so the bubbles don’t erupt too vigorously. You’ll see the pale pink fizz rise up, and the aroma of raspberry and lemon will hit your nose immediately.

Garnish and serve. Add a sprig of fresh mint and a couple of whole raspberries to each glass. Give it a gentle stir and serve straight away, while the fizz is still lively and the mint is fragrant.