1 cup frozen blackberries, raspberries, or a mix, thawed, with whatever liquid accumulates
1/2 cup water
1 tsp lemon juice
2 Tbsp granulated sugar
Pinch of salt
Combine the berries, water, and lemon juice in a food processor or blender. Process until smooth. Force through a sieve and discard the seeds. Add the sugar and salt and stir until completely combined, about 90 seconds. Pour into a 8x8x2″ baking dish. Freeze for half an hour, then stir vigorously with a fork, mashing up any bits that have coalesced. Freeze for another half hour and repeat. Continue doing this for about three hours.
Or, if you don’t want to babysit your freezer, pour the mixture into ice cube trays. Let sit in the freezer until solid (or as close to solid as they’ll get), then pop out of the tray and process in a food processor or blender until finely chopped.