Hull and if necessary wash the fruit. Blend with 150g of the sugar, then pass through a fine sieve and add lemon juice to heighten the taste if required. Prepare 10 individual soufflé dishes (9cm diameter x 6cm deep) by tying a piece of greaseproof paper around the edge to form a collar that stands 5cm above the rim.
Put the egg whites into the bowl of an electric mixer. In a perfectly clean saucepan, dissolve the remaining 250g of sugar in the water over low heat. When the sugar has completely dissolved, bring to the boil, skim off the foam and cook to 120°C/250°F on a sugar thermometer. Beat the egg whites until foamy, then, with the whisk still running, pour the hot sugar directly on to the egg whites, avoiding the beaters. Continue beating until the meringue is cool.
Whip the cream until soft peaks form and fold into the fruit pulp. Delicately fold in the meringue, then spoon into the prepared soufflé dishes. Freeze for 12 hours.
Decorate with fresh berries and, if you like, serve with a sauce made by puréeing 500g berries with 100g caster sugar, sharpening the taste with a little lemon juice.