How to Make It
Hull the strawberries and slice them lengthwise 1/4 inch thick. In a large bowl, toss together the strawberries, boysenberries, raspberries, kirsch and 2 tablespoons of the sugar. Add a little more sugar if desired, depending on the sweetness of the berries (remember that the topping will also add sweetness).
Place a heatproof bowl over a saucepan of simmering water; do not let the bottom of the bowl touch the water. Add the egg yolks, Champagne, and 4 tablespoons sugar and whisk together until the mixture holds its shape when the whisk is raised, about 6 minutes. Do not overcook or the eggs will scramble. Remove the bowl of sabayon from the heat and place it in a bowl of ice water to cool, whisking occasionally.
In a medium bowl, whip the heavy cream to soft peaks. Fold the whipped cream into the sabayon.
Preheat the broiler. Divide the berries among six 5-inch gratin dishes and set the dishes on a baking sheet. Spread 2 tablespoons of the sabayon over each serving. Sprinkle the remaining 4 tablespoons sugar over the gratins. Slide the baking sheet of gratins under the broiler and cook for 3 to 4 minutes, just until the tops turn golden brown. Watch carefully; turn the baking sheet if necessary. Serve at once.