Pour the champagne into a large mixing bowl or container. Add the simple syrup and fresh lemon juice, stirring gently to combine evenly.
Place the mixture in the freezer and let it chill for about 2 hours, occasionally stirring every 30 minutes to break up ice crystals and promote a slushie texture.
Once fully frozen and semi-icy, transfer the mixture into a blender. Blend on high until smooth and slushy, with a creamy consistency that slides easily off the blades.
Pour the blended champagne slushie into chilled glasses, filling them generously to showcase their bubbly, icy appearance.
Garnish with fresh berries or citrus slices on the rim for a colorful, inviting finish.
Serve immediately with a straw or small spoon, enjoying the refreshing, effervescent texture of this elegant summer treat.