Ready time: 3 hours, 15 minutes
3 Granny Smith apples, skins on and quartered
1 cup loosely packed baby spinach
1 large rib celery
3 romaine lettuce leaves
1 slice lemon, skin removed
Juice all ingredients in an electric juicer according to manufacturer’s instructions.
Alternatively, use a blender or food processor to break down ingredients in batches, then press through a fine-mesh strainer to separate juice from pulp.
Pour juice into a shallow, freezer-safe container.
Freeze for about 3 hours, removing container from the freezer every half hour or so to scrape and break up ice.
Serve granita in an icy mound with a piece of candied lemon peel or fresh mint as garnish.