Pop the watercress into a food processor and wizz down into a paste.
Add the other ingredients and whizz again until all is combined. At this point you really need to taste it, if it is a little bland add salt & pepper & whizz again.
If it's still not singing add a little more lime juice, anchovies and/or garlic to pull out the flavour you particularly want. There is no right or wrong, just keep tasting until it makes your mouth a little bit excited to taste :)
Scrape all the salsa verde out into a small bowl and serve, or cover with cling film and pop in the fridge until needed.