Wash the basil leaves and pat them dry.

Spread the basil leaves out on a clean surface or newspaper to air dry.

Roast the pine nuts or walnuts in a dry frying pan over medium heat until lightly toasted and fragrant.

Place the roasted pine nuts or walnuts, garlic cloves, Parmesan cheese, and salt into a food processor.

Pulse the ingredients in the food processor until coarsely chopped.

With the food processor running, slowly drizzle in half of the olive oil.

Process the mixture until it starts to become smooth.

Add the basil leaves in three batches, pulsing the food processor after each addition.

Process the mixture until the basil leaves are finely chopped and incorporated.

Slowly pour in the remaining olive oil while the food processor is running.

Continue to process the pesto until it reaches a smooth and emulsified consistency.

Transfer the pesto to a clean container.

Pour a thin layer of olive oil over the top of the pesto to prevent oxidation.

Store the pesto in the refrigerator or freezer.