Blend the berries with 150g/5½oz of the sugar in a blender, then pass through a fine sieve and add lemon juice to taste.
Prepare 10 x 9x6cm/3½x2⅓in individual soufflé dishes by tying a piece of greaseproof paper around the edges to form a collar that stands 5cm/2in above the rim.
Dissolve the remaining 250g/9oz of sugar in 250ml/9fl oz of water in a saucepan over a low heat. When the sugar has completely dissolved, bring to the boil, skim off the foam and cook to 120C, testing with a sugar thermometer.
Whisk the egg whites until foamy in a stand mixer, then pour the hot sugar directly onto the whites while whisking, avoiding the beaters. Continue beating until the meringue is cool.
Whip the cream until soft peaks form. Fold it into the meringue along with the berry purée. Spoon into the prepared soufflé dishes. Cover and freeze for 12 hours.
Remove the collars and decorate with fresh berries to serve.