Place half the lemon juice in a large bowl. Halve and core apples. Thinly slice. Add apple to lemon juice (see top tip). Gently toss to coat.
Combine mayonnaise and remaining lemon juice in a bowl. Season with salt and pepper. Thinly slice celery stalks. Chop leaves.
Add celery, celery leaves and beetroot to apple mixture. Gently toss to combine.
Tossing sliced apple in lemon juice prevents the apple turning brown.
*Although not traditionally counted; beetroot is providing the 1g protein per serve