Make the lemon syrup. Using a vegetable peeler, peel the zest from 3 of the lemons in wide strips, being careful to avoid the white pith, which will add a bitter, chalky note. Place the zest strips in a small saucepan with the caster sugar and 250 ml of water. Set the pan over a medium heat and stir gently until the sugar dissolves completely—you will see the liquid turn from cloudy to completely clear, and the air will fill with a sharp, sweet citrus perfume. Bring it to a gentle simmer, then reduce the heat to low and let it bubble softly for 8 minutes. The syrup will thicken slightly and take on a pale golden hue. Remove from the heat and let it cool completely in the pan; the zest will continue to infuse the syrup.

Prepare the watermelon. While the syrup cools, cut the watermelon in half. Use a large spoon to scoop the flesh into a bowl, working around the seeds as best you can. You should have about 1 kg of flesh. Working in batches, place the watermelon in a blender and blitz on high for 30 seconds, until it becomes a smooth, frothy liquid. You will see a light pink foam form on top. Pour the puree through a fine-mesh sieve into a large jug, pressing gently on the solids with the back of a spoon to extract every last drop of juice. Discard the pulp. You should have about 600–700 ml of pure juice, a vivid, almost neon pink.

Juice the lemons. Roll the 3 zested lemons (plus the 1 extra lemon) firmly on the countertop under your palm to break the internal membranes—this helps release more juice. Cut them in half and juice them into a small bowl. You need about 120 ml of fresh lemon juice. The colour should be a bright, sunny yellow, and the smell should be tart enough to make your mouth water.

Combine the base. In a large pitcher, whisk together the watermelon juice, the fresh lemon juice, and the cooled lemon syrup (discard the zest strips first). The mixture will be a deep, rosy coral. Add the 500 ml of cold water and stir gently. Taste the lemonade at this point—it should be sweet, tart, and intensely fruity. If it feels too concentrated, add another 50–100 ml of cold water.

Chill and rest. Cover the pitcher and place it in the refrigerator for at least 30 minutes. During this time, the flavours will meld and soften. The colour will deepen slightly, and the sugar from the syrup will fully integrate into the liquid, creating a smooth, unified body.

Serve with ceremony. Fill four tall glasses with ice cubes. Pour the chilled lemonade over the ice, leaving about 2 cm of space at the top. Garnish each glass with a thin slice of lemon and a sprig of fresh mint, if using. The mint adds a cool, herbaceous note that lifts the drink beautifully. Serve immediately, and watch the condensation bead on the glass.