Step 1

Place the basil leaves and garlic in a food processor or blender, and mix well. Add the walnuts, and continue to blend until the nuts are finely ground.

Step 2

Keep the machine running as you drizzle in the olive oil. When you have a smooth paste, transfer to a bowl, and stir in the parmesan. Season to taste with salt and pepper.

Step 3

If you are not going to use the pesto right away, place in a container that has a tight-fitting lid. Smooth out the top surface of the pesto, and pour on enough olive oil to cover it completely. This creates an air-proof seal that helps preserve it well. Cover tightly and store in the refrigerator.