Tip the hulled strawberries into a small pan with water. Cover and simmer for 20 minutes until the strawberries are soft. Strain the juice from the fruit (discard the berries) and measure the collected juice – you should have 350ml/12fl oz,make up the quantity with water if necessary.
Return the juice to the pan, add the caster sugar and heat until dissolved. Let it cool, then freeze and whisk as for the coffee granita.