In a small saucepan, mix the blackberries, sugar, Grand Marnier, and lemon zest. Simmer over medium heat until the juices are released and the sugar is dissolved, stirring frequently, about 5 minutes. Strain, pressing with the back of a spoon to force the pulp through the strainer. Discard the seeds. Add the raspberries and the smashed strawberries to the hot mixture, pressing gently on the raspberries to crush them slightly. You'll have about 1-1/3 cups of berry mixture. Chill until very cold, about 3 hours. Taste and add more sugar, liqueur, or zest if needed, remembering that the flavors will be muted when you fold the puree into the cream.

In a chilled bowl, whip the cream to firm but not stiff peaks. With a rubber spatula, gently fold 1 cup of the chilled berry mixture into the whipped cream just until incorporated. Serve immediately or refrigerate up to 24 hours. Just before serving, drizzle the remaining berry mixture over the fool.