Put egg yolks and sugar in a heatproof bowl and whisk until well combined. Add Champagne and liqueur and beat well. Place bowl over a pot of simmering water and whisk vigorously for 5 minutes until mixture has a thick, foamy consistency.
Arrange sliced fresh fruits decoratively on a serving plate. Pour sabayon over fruits such as strawberries, plums etc., place under a hot grill close to flame to brown quickly.