In a blender, puree the mixed berries (yields about 5 1/2 cups). Soften gelatin by sprinkling it over 1/4 cup cold water. In a saucepan over high heat, combine half the berry puree with sugar and softened gelatin. Heat until all sugar has dissolved and gelatin has melted.Advertisement
Transfer berry mixture to a large bowl and place over an ice bath until cool, then add remaining berry puree.
Beat heavy cream in the bowl of a mixer fitted with the whisk attachment until medium-stiff peaks form, 6 to 8 minutes. Fold whipped cream into berry mixture in 4 additions.