Pour the champagne into a measuring cup or bowl and add the fresh lemon juice along with the simple syrup.
Mix gently with a spoon or whisk until well combined, allowing the citrus to blend smoothly into the bubbly.
Transfer the mixture to a shallow baking dish or a freezer-safe container.
Place the container in the freezer and let it chill for about 2 hours, stirring every 30 minutes to break up ice crystals and promote an even slushie texture.
Once the mixture is frozen and reaches a slushy consistency, use a fork to scrape and fluff the frozen surface, creating a frosty, textured drink.
Spoon or scoop the champagne slushie into chilled glasses and serve immediately to enjoy its refreshingly icy and bubbly quality.