Whisk egg whites until soft with an electric or rotary beater.
Add the sugar slowly and continuously, beating all the time.
The mixture will become a smooth, glossy meringue.
Whisk in vanilla and vinegar and stop beating.
Fold in chopped hazelnuts with a tablespoon or plastic spatula.
Divide mixture onto two 8 inch pie plates, bottom-lined with rice paper or silicone paper, lightly greased and floured, and smooth tops.
Bake in a moderate oven for 30-40 minutes.
Transfer carefully to a wire rack to cool.
Whip cream, clean raspberries or slice strawberries.
Sandwich cold meringues together with three quarters of the whipped cream and prepared fruit.
Sift a little icing sugar over top and pipe rosettes of cream around edge.
Let stand in a cool place for 3 hours before serving so that gateau is easier to cut into segments.