Place walnuts, pine nuts, and garlic in the bowl of a food processor fitted with a steel blade. Process for 30 seconds.
Add the basil leaves, salt and pepper. With the processor running, slowly pour the olive oil into the bowl through the feed tube and process until the pesto is finely puréed.
Add the Parmesan and purée for a minute. Serve, or store the pesto in the refrigerator or freezer with a thin film of olive oil on top.