Blend the watermelon. Place the cubed watermelon into a blender. Blitz on high for about 30 seconds until you have a smooth, vibrant pink liquid with no visible chunks. The colour should be bright and uniform.

Strain the juice. Pour the blended watermelon through a fine-mesh sieve into a large jug. Use the back of a spoon to press the pulp, extracting every drop of juice. You'll be left with a clear, ruby-red liquid and a small amount of dry pulp in the sieve.

Add the lemon and sweetener. Stir in the fresh lemon juice and honey. Whisk gently until the honey has completely dissolved — the mixture will look slightly lighter in colour and smell intensely sweet-tart. Taste it here; it should be bright and balanced.

Chill the base (optional but recommended). If you're making this ahead, place the jug in the fridge for up to 4 hours. The liquid will stay a deep pink and develop a slightly thicker texture as it cools.

Add the fizz. Just before serving, pour the chilled sparkling water into the watermelon mixture. Stir very gently with a long spoon — you want to keep the bubbles intact. You'll hear a soft fizz and see tiny bubbles rising to the surface.

Fill the glasses. Divide the ice cubes evenly among 4 tall glasses. Pour the mocktail over the ice, leaving about 2cm of space at the top. The ice will crackle softly as the liquid hits it.

Garnish and serve. Slap a mint leaf between your palms to release its aroma, then float it on top. Add a thin lemon slice to the rim of each glass. Serve immediately while the bubbles are lively.