Thoroughly wash all produce under cool running water.
Remove stems from kale leaves and tear into smaller pieces.
Roughly chop celery into 1-inch pieces.
Peel the ginger using the edge of a spoon.
If using apple, core and chop into chunks (leave skin on).
Add kale, celery, ginger, and apple (if using) to your blender.
Squeeze in the juice from a lemon.
Pour in the filtered water.
Blend on low speed initially, then increase to high for about 60 seconds until smooth.
For a smoother texture, strain the mixture through a fine-mesh sieve or cheesecloth.
Press with the back of a spoon to extract all liquid.
Pour into a glass over ice and serve immediately.
