In a small saucepan, combine the water and sugar and heat gently, stirring until fully dissolved without boiling, then remove from heat, add the lemon zest, cover, and let steep for 10–15 minutes before straining and cooling completely.
Add the lemon pulp, fresh lemon juice, 120 ml of the peel-infused syrup, basil leaves, the pinch of fine sea salt, ice-cold water (starting with 1½ cups), and the ice cubes to a high-speed blender.
Blend on high until fully smooth and slushy, then taste and adjust by adding more water if too thick, more ice if too thin, and more syrup only if needed; add basil cautiously — once it dominates, you cannot undo it — and aim for a final total water volume of about 4 cups, counting melted ice.
Pour into chilled glasses and garnish with lemon slices and fresh basil leaves.
