Place the 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 thoroughly pureed. Add the Parmesan and puree for a minute. Serve, or store the pesto in the refrigerator or freezer with a thin film of olive oil on top.