Place cilantro leaves, walnuts and garlic in the bowl of a food processor. Process for around 20 seconds.
Continue to process whilst slowly pouring in the olive oil.
Scrape the sides of the bowl with a spatula and process for a few more seconds, repeat if necessary until the cilantro has been blended up.
Add the parmesan and lemon juice and process for 20 seconds.
Season with salt and pepper and pulse, adjust until the seasoning is correct.