In a small saucepan, combine the white sugar and 1 cup (240 ml) water. Heat gently over medium heat, stirring just until the sugar fully dissolves and the liquid becomes clear. Do not boil — excessive heat dulls perceived freshness and flattens citrus brightness. Remove from heat and allow the syrup to cool completely before using.

Juice 5 medium lemons to obtain about 220–250 ml fresh lemon juice. Strain out seeds but keep the juice natural and vibrant. Taste the juice — it should feel sharp and clean, not bitter or oxidized.

In a large pitcher, combine the fresh lemon juice with about two-thirds of the cooled simple syrup. Stir well to fully integrate. The base should taste slightly more intense than the finished drink, since carbonation and dilution will soften both sweetness and acidity.

Add the ice-cold sparkling water and the pinch of fine sea salt. Stir very gently once or twice only — just enough to combine while preserving carbonation. The drink should immediately feel lively, crisp, and refreshing.

Taste and adjust deliberately. Add a small amount of remaining syrup only if acidity feels too aggressive. Avoid adding extra lemon juice at this stage — structure is already set and carbonation will continue to sharpen perception.

Fill glasses with ice, pour the sparkling lemonade, garnish with lemon slices, and serve right away while fully cold and effervescent.