Place the garlic in the bowl of a food processor fitted with a steel blade. Process for 30 seconds until finely chopped. Add the walnuts and process until roughly chopped, another 30 sec. Add the basil leaves, salt, and pepper (pulse a few times to make it all fit, if needed). 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 cheese and puree for a just a second to mix. Serve, or store the pesto in the refrigerator or freezer with a thin film of olive oil on top.