Chop the tomatoes and place in a bowl with all the other ingredients except the basil leaves. If you have time, leave them to macerate at room temperature overnight. Blitz in a blender - in batches, then strain through a fine sieve. Shred the basil leaves and stir through.
Clear a shelf in the freezer and place a sided
metal tray in to chill. Make up your mix and leave to cool, then
pour into the tray and cover with cling film.
Freeze the liquid until frozen around the edges
and slushy in the middle. Use a fork to break up
the ice into smaller
crystals. Return the tray to
the freezer. Repeat the
breaking up process
every half hour (at least
three times) until it’s completely frozen and
the texture of snow. For a quicker method: freeze the mixture
in an ice tray, then blitz the cubes in a food
processor just before serving. The texture
will be slightly slushier, but just as delicious.