Place clean spinach, kale, or basil into bowl of food processor along with nuts and garlic, and process until well chopped and combined. Scrape down sides of bowl and ,if necessary, pulse once or twice more.

With motor running, drizzle olive oil into the chopped spinach and blend until emulsified, about a minute. Scrape down sides, add the salt, and pulse twice more to mix.

If you want to freeze your pesto for later use, stop at this point of the recipe and store your pesto in a small, well-sealed container in the refrigerator or freezer. When you are ready to serve your pesto, stir in the cheese and lemon zest.