Put 2 cups of fresh basil leaves, a garlic clove, 1/4 cup of pine nuts and 1/2 cup of olive oil in a blender or food processor. Puree the ingredients until they form a paste. Add salt, pepper and freshly grated Pecorino or Parmesan cheese, and process just until blended.
The ingredients in traditional pesto sauces are often crushed using a mortar and pestle, but a blender or food processor makes the job easier. Blenders often result in a smoother sauce, because the ingredients gravitate toward the blades instead of bouncing around in a food processor.
Wash the basil leaves well before making the pesto sauce. Dirt left on the leaves makes the sauce grainy. Cool water is best so that the leaves don't wilt. Dry the leaves with a clean cloth to remove excess water.
While basil is the traditional pesto ingredient, other leafy greens work in its place. Some options include parsley, arugula, kale and cilantro. Replacement options for the pine nuts include walnuts, pecans and almonds. Roasting the nuts lightly before making the pesto sauce adds flavor and texture. Wait until the nuts cool completely before adding them to the mixture.
Any pesto sauce that is not used right away should be refrigerated. Press plastic wrap on top of the sauce before refrigerating.