Choose the ripest and sweetest berries available.
Sprinkle 2 tablespoons sugar and 2 tablespoons sherry over berries; let stand, tossing gently once or twice, for about 30 minutes.
Combine remaining 3 tablespoons sugar, 1/3 cup sherry, and egg yolks in a small bowl; whisk together. Set bowl over pan of simmering water, and continue whisking until tripled in volume, frothy, and warm to the touch, 6 to 8 minutes. Serve berries and sabayon together.