Wash the basil leaves and dry them thoroughly with paper towels
Chop the garlic, removing any green shoots in the heart of the clove.
Place the garlic and basil in a food processor or blender.
Add the pine nuts and pour in about one-third of the oil.
Process or pulse, stopping occasionally to press the leaves down with a wooden spoon and to add oil, until the pesto has a slightly chunky texture with some whole pine nuts left.
Do not overheat the pesto by excessive blending. Spoon the pesto into a ceramic or other non-metallic bowl.
Stir the Parmigiano-Reggiano into the sauce and add salt to taste.
Cover the pesto with a film of olive oil until ready to use.
Note: If you are planning to store pesto for any length of time, do not add the cheeses or garlic, since these ingredients slowly ferment; instead add them just before using the sauce. To store pesto, place it in a glass jar or earthenware container—never use metal or plastic containers or utensils. Cover the surface of the pesto with a thin film of olive oil to prevent oxidation and discoloration and seal tightly. Pesto can be kept in the refrigerator for several weeks if the garlic and cheese have not been added, or it can be frozen for several months.