
If you don't know what that means, get a large bowl and add a few cups of ice cubes to it. Get another bowl that's small enough to nest inside the large bowl and set it inside. Pour a bit of cold water into the larger bowl, taking care not to over-fill it—you DON'T want water to get into the center bowl. Pour the grape gelatin mixture into the center bowl and whisk it. It will cool down very quickly.
Once the grape mixture has cooled to room temperature, remove the center bowl from the ice bath and *gently* pour the grape mixture on top of the custard in the tart. Place the tart in the refrigerator and chill for 4 hours to solidify the grape gel. Before serving, garnish with fresh fruit. Serve chilled.