Make the simple syrup. In a small saucepan, combine the caster sugar and 100 ml of water. Set over a medium heat and stir gently with a wooden spoon. You'll hear the sugar granules scraping against the pan at first, then a soft silence as they dissolve. Once the liquid is completely clear and you can smell a faint, sweet warmth, remove it from the heat. Let it cool to room temperature. The syrup should feel silky between your fingers, not gritty.

Prepare the watermelon. Place your watermelon chunks into a high-speed blender. You want to see the deep pink flesh tumble into the jug. Blitz on high for about 30 seconds. The sound will start as a loud, chunky thudding, then mellow into a smooth, whirring hum. You'll see the colour change from solid pink to a vibrant, almost neon liquid. The aroma at this point is intoxicating — pure, sweet melon air.

Strain the juice. Place a fine-mesh sieve over a large bowl. Pour the blitzed watermelon through it. Use the back of a ladle or a spatula to gently press the pulp against the mesh. You will feel the resistance of the fibres, and you'll see the clear, ruby-coloured juice pooling below. You should have roughly 500-600 ml of juice. Discard the dry, fibrous pulp left in the sieve. The texture of the juice should be perfectly smooth, with no seeds or stringy bits.

Juice the lemons. Roll the lemons firmly on the countertop with your palm to soften them. Cut them in half and juice them. Listen for the squelch of the flesh and the patter of juice hitting the bowl. You need about 120 ml of bright, tart lemon juice. Strain it to remove any pips. The smell will be sharp and clean, a perfect counterpoint to the sweet melon.

Combine and chill. In a large jug, combine the strained watermelon juice, the fresh lemon juice, the cooled simple syrup, and a generous pinch of sea salt. Stir well with a long spoon. Taste it. It should be a perfect balance of sweet, tart, and a whisper of savoury from the salt. If it's not sweet enough, stir in the optional agave or honey. Cover the jug and place it in the fridge for at least 1 hour. This step is crucial — the flavours need time to marry and deepen. The jug will feel cold to the touch when it's ready.

Assemble the mocktails. When you are ready to serve, take four tall glasses. Add a few ice cubes to each one. The ice should clink cheerfully against the glass. Gently tear a few mint leaves and drop them into each glass. You'll catch the fresh, peppery scent of the mint as it touches the ice.

Add the fizz and serve. Fill the glass about two-thirds full with the chilled watermelon-lemon mixture. Then, top it up with the chilled sparkling water. Watch as the bubbles race upwards, creating a beautiful, fizzy crown. Stir once gently with a straw. Garnish each glass with a thin lemon slice and a sprig of mint. The final drink should be cold, effervescent, and a deep, sunset pink. Serve immediately. My husband, who's usually picky, asked for seconds the first time I made this, and now he hovers near the kitchen whenever he sees a watermelon on the counter.