Wash the watercress and parsley, removing any tough stalks and roughly chop. Place into a food processor along with the cashews, garlic and lemon juice and pulverize. With the machine going drizzle in olive oil to create a paste. The quantity of olive oil will depend on the consistency desired. Adjust seasoning to taste. Use as a spread on sandwiches, stir through pasta, spoon into soup.
Notes
TIP: The trick to keeping the pesto from going brown is to use plenty of olive oil while blending. The oil coats the cut leaves so to protect them from oxidation.