How to Make It
Preheat the oven to 350°. Put the walnuts in a pie plate and toast for about 8 minutes, or until golden and fragrant. Let cool completely.
In a food processor, pulse the arugula leaves with the toasted walnuts and smashed garlic cloves until finely chopped. Add the shredded Manchego cheese, lemon zest and juice and pulse until combined. With the machine on, add the olive oil in a steady stream and process to a smooth paste. Season the pesto with salt and pepper, transfer to a bowl and serve.