Add the chilled lemon juice, lemon zest, and sugar into a blender. Blend on low for about 30 seconds until the sugar begins to dissolve and the mixture is fragrant.
Pour in the cold water to loosen the mixture slightly, then turn the blender to high speed.
Drop in the ice cubes all at once and blend vigorously for 1-2 minutes, or until the mixture becomes thick, slushy, and has a smooth, icy texture.
Stop the blender and check the texture; it should be cold, thick, and slightly granular from the ice. If needed, blend a few seconds longer to achieve the perfect slush consistency.
Taste the slushie and adjust sweetness if necessary by adding a splash of honey or more sugar, then briefly blend again to incorporate.
Pour the chilled lemon slushie into glasses, and garnish with a small lemon wedge or additional zest if desired. Serve immediately for the best icy crunch.
