Add raspberries, sugar, and water to a saucepan.
Cook on medium heat for about 8 minutes, stirring gently, until berries break down and sugar dissolves.
Remove from heat and let it cool slightly.
Blend the mixture until smooth.
Strain through a fine sieve to remove seeds.
Stir in lemon juice.
Chill the mixture completely in the refrigerator.
Slowly stir in the chilled champagne.
Pour into a shallow freezer-safe container.
Freeze for 30 minutes, then stir well.
Repeat stirring every 30 minutes for about 3 hours, until smooth and scoopable.