Juice the lemons using a citrus juicer until you have about one cup of fresh lemon juice, then pour it into the blender.
Add elderflower syrup to the lemon juice, measuring it carefully to balance floral sweetness with tartness.
Tear fresh mint leaves roughly with your fingers to release their aromatic oils, then add them to the blender.
Pour cold water into the blender to help the mixture blend smoothly and to dilute slightly for a refreshing taste.
Add ice cubes to the blender, filling it up to the top, to create a chilled, slushy texture.
Secure the lid on the blender and blend on high until the mixture is smooth, icy, and well combined, about 30-45 seconds.
Pour the slushie into glasses, ensuring each has plenty of crushed ice and bright flavor.
Garnish with extra mint leaves or a lemon wedge if desired for an attractive presentation.