Add all of the ingredients to a high speed blender. Process until bright green, smooth and creamy (sauce should be the consistency of sour cream or a thin mayonnaise); add water as necessary to thin to desired consistency, season with salt and pepper.
Store in an airtight container in the fridge, for up to 1 week.
Notes
Add more water to make a thinner dressing, less or none at all for a thicker, more dip-like consistency.