In a cocktail shaker filled with ice, combine the gin, cranberry simple syrup, and fresh lemon juice.
Shake well until the mixture is well chilled.
Strain into a champagne flute.
Carefully pour the Champagne into the flute.
Garnish with fresh cranberries.
Notes
Prepare the Cranberry Simple Syrup: In a saucepan, combine equal parts sugar and water with a handful of fresh cranberries. Bring to a simmer until the cranberries burst and the sugar is dissolved. Let cool, then strain the syrup and discard the solids.