Juice the lemons until you have about one cup of fresh lemon juice. Strain to remove seeds and pulp if desired for a smoother slushie.
Add the lemon juice to a blender, then pour in the simple syrup, adjusting the sweetness as preferred.
Pour in the ice cubes, filling the blender almost to the top to ensure a thick, icy texture.
Secure the lid tightly and blend on high speed until the mixture becomes slushy and smooth, with a fine icy consistency that is spread evenly and looks frosty.
Optional: add a few fresh mint leaves for a refreshing aroma or splash in sparkling water for extra fizz, then briefly blend again to combine.
Pour the lemonade slushie into chilled glasses, revealing a bright, icy mixture with a slightly granular, crushed-ice appearance.
Garnish with a sprig of mint or a lemon wedge if desired and serve immediately to enjoy its refreshing, icy texture.