How to Make It
In a large heatproof bowl set over a pot of simmering water, whisk the egg yolks with the 1/4 cup of sugar, the liqueur and lemon juice over moderate heat until thick and pale and an instant-read thermometer inserted into the mixture registers 155°, about 7 minutes. Remove the bowl from the saucepan and let the sabayon cool.
In a large bowl, using a handheld electric mixer, whip the heavy cream until firm. Fold the whipped cream into the sabayon and refrigerate until chilled, about 1 hour. In another bowl, toss the berries with the remaining 1 teaspoon of sugar and let stand for 10 minutes. Spoon the berries into bowls, top with the sabayon and serve.
St-Germain elderflower liqueur is available at many liquor stores or online from klwines.com.