Whisk the gelatin and boiling water in a medium bowl until the gelatin is dissolved, about 2 minutes. Refrigerate until lukewarm, about 25 minutes. Stir in the Prosecco. Cover and refrigerate until the gelatin thickens but does not set, stirring occasionally, about 1 hour. Stir in the raspberries (bubbles will form in the gelatin mixture). Divide the gelatin among 6 Champagne flutes or individual dessert bowls. Cover and refrigerate until the gelatin is set, at least 2 hours and up to 2 days.
Dollop the mascarpone cheese atop each serving, then sprinkle the walnuts over and serve.