1If you’re using kale, bring a small pot of salted water to a boil; add all the greens and blanch for 2-3 minutes, until tender and wilted. (You can do this with other greens, but it isn’t as necessary; more tender greens can be tossed into the pesto raw.)
2Drain the greens well in a colander and set aside to cool. Place in the bowl of a food processor with the walnuts, Parmesan, garlic and lemon juice and pulse until well mulched. With the motor running, pour in the olive oil, blending and scraping down the side of the bowl, until well blended and roughly smooth. Season with salt and pepper.
3Serve immediately or refrigerate in a jar for up to a week.