1. Bring a small pot of water to a boil and place a bowl of ice water next to the stove. Drop the basil leaves into the boiling water, just for a few seconds until they wilt. Immediately remove them with a spider or slotted spoon and immerse them in the ice water. Lift the leaves and transfer them to a colander to drain. Gently squeeze out excess water and let them stand briefly between paper towels to absorb excess moisture.
2. Finely chop the garlic in a food processor. Add the basil leaves, nuts, the ¼ teaspoon of salt and the ⅛ teaspoon of pepper and process until the ingredients won’t break down any further. With the motor running, pour ½ cup of olive oil through the top feed tube and blend until the mixture is smooth and creamy. Scrape down the side of the bowl, add the parmesan, and blend briefly to incorporate it. Adjust the salt and pepper to taste. Add more oil—up to ¼ cup—to thin the pesto if you wish.
The pesto will keep, in an airtight container in the refrigerator, for up to 3 days. You can also freeze it in ice cube trays or an airtight container for up to 6 months.