In a food processor add pine nuts, basil, garlic cloves, lemon zest, lemon juice, salt, pepper, and parmesan cheese. Pulse until everything is blended finely.
Scrape down the sides.
While the food processor is running, slowly add the olive oil in a steady small stream. Slowly adding the olive oil will cause the pesto to emulsify, and will also help keep the olive oil from separating.
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