Separate the celery stalks, ensuring they are washed thoroughly to remove any dirt. Trim off the root ends and roughly chop the stalks, reserving one tender, young piece to use as garnish when serving.
In a blender or food processor, combine the chopped celery, apple, ginger, and half of the lemon juice. Pulse until the mixture is finely chopped. With the motor running, gradually add 50–100ml of cold water and continue blending until you achieve a smooth consistency.
Pour the blended mixture through a fine mesh sieve into a jug, pressing down with the back of a spoon to extract as much juice as possible. Cover and chill the juice, or serve immediately for the freshest flavor. Stir well before pouring into glasses. Add an extra squeeze of lemon juice to taste, if desired, and garnish with the reserved celery stalk for an elegant touch.