Blanch the parsley for 3 minutes in a pot of boiling water. Meanwhile, prepare a bowl of ice water. Transfer the parsley to the ice water to cool it. Drain the parsley, squeezing with your hands. In a food processor, purée the parsley and the very cold water. Proceed quickly to avoid overheating the purée (otherwise the sauce will lose its beautiful bright colour). Strain through a sieve place over a bowl. Collect 1/2 cup (125 ml) of this green juice for the recipe. Keep in the refrigerator.