Puree grapes in food processor or blender.

Strain grape puree through a fine-mesh sieve set over a 13x9-inch baking dish, pressing solids with the back of a large spoon. Discard peel.

Add champagne, sugar, and lemon juice, and slowly stir until sugar dissolves.

Cover with plastic wrap and freeze for 3 to 4 hours, or until frozen, stirring every hour.

Remove and scrape with a fork until fluffy.

Scoop into serving bowls and garnish with grapes.

Serve.