Bring a large pot with water to a rolling boil. Add 100g / 3.53oz of salt for every liter of water.
Blanch the parsley leaves for 10 seconds, stirring them gently so that they all get evenly blanched; the leaves will appear wilted once they are fully blanched but will still be bright green.
Shock the leaves in an ice bath, stirring so that they chill quickly.
Strain the leaves out of the ice bath. Squeeze the excess water off with your hands and pat dry with paper towels.
Place half of the leaves in a blender with the oil. The oil should cover the leaves. Add more oil if necessary.
Blend on medium speed for about 1 minute or until smooth, then add the remaining parsley and the xanthan gum and blend for 1 more minute. If the parsley is not blending properly, add a little more oil.
Once a smooth purée has been obtained, remove from the blender and strain through a fine-mesh strainer. Reserve only the pulp that remains in the strainer; the remaining liquid will not be used. Refrigerate until needed. Discard after 2 days.