Roll the lemons on the countertop to loosen the juice inside, then slice in half and juice until you have about one cup of fresh lemon juice.
Use a zester or fine grater to carefully remove the yellow zest from one lemon, avoiding the bitter white pith, then set aside.
Measure out your syrup or sweetener; add a tablespoon or two to the lemon juice, adjusting for sweetness to taste, and stir to combine.
Place the ice cubes into your blender, then pour in the lemon mixture and sprinkle over the lemon zest.
Blend on high until the mixture is smooth, frosty, and has a slushy texture, about 30-45 seconds.
Pour the vibrant yellow slushie into glasses, garnish with additional lemon zest if desired, and serve immediately for best texture.
