How to Make It
Bring a large saucepan of salted water to a boil and fill a bowl with ice water. Add the herbs to the saucepan and blanch for 15 seconds. Drain and immediately transfer the herbs to the ice water to cool. Drain, gently squeezing out the excess water.
Transfer the herbs to a blender. Add the cold water and puree until smooth. With the blender on, add the xanthan gum and puree until slightly thickened, about 15 seconds. Blend in the lemon juice and season with salt and white pepper. Use the sauce right away.