To make the dressing, blend the grapes in a blender until pureed. Pour into a sieve over a jug, pressing as much liquid through as possible. Discard the solids then pour into a serving jug. Whisk the oil, vinegar and mustard in a separate jug until well combined.
Use a mandolin to thinly slice the apples into rounds. Squeeze the lemon halves over the apple slices to prevent browning.
Arrange the lettuce wedges on a board or platter. Top with apple slices, celery and grapes. Spoon the oil dressing over then spoon over the grape juice. Sprinkle with walnuts. Season to serve.