2) 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.
3) Add the Parmesan and puree for a minute. Use right away, or store the pesto in the refrigerator or freezer with a thin film of olive oil on top.
Cook's Notes: Air is the enemy of pesto. For freezing, pack it in containers with a film of oil on top or cover with cling film with all the air pressed out.
To clean basil, remove the leaves, swirl them in a bowl of water, and then spin them very dry in a salad spinner. Store them in a closed plastic bag with a slightly damp paper towel. As long as the leaves are dry they will stay green for several days.