2/3 cup [160 milliliters] extra-virgin olive oil, plus more for storing
1/2 cup [40 grams] freshly grated Parmigiano-Reggiano cheese
In a food processor fitted with a steel blade, put the nuts and garlic and process until coarsely chopped, about 10 seconds.
Add the basil, salt, and pepper and process until the mixture resembles a paste, about one minute.
With the processor running, slowly pour the olive oil through the feed tube and process until the pesto is thoroughly blended. Add the Parmigiano-Reggiano and process one minute more.
Use the pesto immediately or store in a tightly sealed jar or airtight plastic container, covered with a thin layer of olive oil (the oil seals out the air and prevents the pesto from oxidizing, which would turn it an unappetizing brown color). The pesto will keep in the refrigerator for about a week or can be frozen for up to three months.