Pour the chilled Champagne into an ice cube tray and freeze until solid, about 4 hours or overnight.
Once frozen, transfer the champagne ice cubes to your blender along with the fruit syrup, lemon juice, and ice cubes.
Pulse the blender gently at first to break up the ice, then blend on high until the mixture is smooth and icy, with a slightly frosty texture and tiny bubbles visible throughout.
Stop the blender and check the consistency; if it’s too watery, add a few more frozen fruit or ice cubes and pulse again until thickened.
Pour the bubbly slushie into glasses and garnish with a slice of lemon or fresh berries if desired.
Serve immediately for the best icy texture, allowing the bubbles to fizz with each sip and enjoy the refreshing, sparkling treat.