Peel a grapefruit raw, on top of a container to collect the juice by removing the white skin. Cut it into thin slices. Cover and place in a cool place.
Rehydrate the gelatin in a bowl of water. Squeeze the juice of the second grapefruit, filter it. Whisk together the cottage cheese with the honey, then whisk in the icing sugar and grapefruit juice.
Whip the (very cold) liquid cream into a firm whipped cream and gently fold it into the mixture. Drain the gelatine, melt it for a few seconds in the microwave, then mix it with the mousse. Cover and place in a cool place.
Whisk the egg whites with the salt, add them gently to the mousse. Divide into bowls, decorate with slices of grapefruit. Cover with cling film and place in a cool place for at least 3 hours.