Traditionally, pesto is a sauce made of fresh basil leaves, garlic, pine nuts, olive oil and aged hard cheese such as Parmigiano-Reggiano and/or Pecorino Sardo.
And strictly speaking, that's what it still is. To a certain extent, however, the notion of pesto has become an archetype, its meaning having expanded to encompass any preparation involving a puree of some leafy green ingredient along with garlic, olive oil, nuts, and cheese.
Now, the interesting thing about extra-virgin olive oil is that it contains relatively large (compared with other oils) amounts of a chemical compound called polyphenols, which usually remain trapped within the fat molecules of the oil.
But when those fat droplets are broken up by the blades of a blender or food processor, the polyphenols, which have a bitter flavor, are released into the emulsion. Thus, the more the oil is blended, the more bitter it can become.
Solution: Puree the basil, garlic and pine nuts in the blender, then stir in the cheese and oil by hand.
Alternate solution: Use pure olive oil instead of extra-virgin olive oil. Pure olive oil has been refined, a process that among other things removes most of the polyphenols, which means blending it will not cause bitterness.
Another solution is to use a different sort of oil altogether, such as walnut oil or avocado oil. Obviously, some oils have a more pronounced flavor than others, so a bland, mild oil will produce a correspondingly bland, mild pesto. (Here's some more info about different oils and fats.)
Similarly the nuts. Walnuts are often substituted for the pine nuts, which aren't exactly cheap. But you can also use cashews, pistachios, almonds, or, I kid you not, pumpkin seeds (aka "pepitas," but make sure they have the hulls removed.)
As for the cheese, it's best to use a hard cheese such as Parmigiano-Reggiano, Pecorino Romano, or the aforementioned Sardo, which can be difficult to find. Alternatively, experiment with other aged hard cheeses (sometimes referred to as "grating cheeses"), including ones made from sheep's milk.
By the way, one nice advantage of adding the olive oil at the end is that it allows you to control the consistency. If you're planning to use the pesto for pasta or gnocchi, you might want it a bit thinner. For a dip or spread, use less oil and it'll be thicker.
You can also freeze pesto. A great trick is to spoon it into ice cube trays, freeze it, then crack them out into a Ziplock bag so that you can use them whenever you want.
- 2 cups fresh basil leaves (about 1 large bunch)
- 3 cloves garlic
- ½ cup pine nuts (or walnuts)
- 3/4 cup freshly grated hard cheese such as Parmigiano-Reggiano or Pecorino-Romano cheese, or a combo of the two
- ½ cup olive oil
- Kosher salt to taste
- Combine all the ingredients except the oil and cheeses in a food processor. Pulse until the pesto is blended into a slightly coarse paste.
- Transfer to a bowl and stir in the oil and cheeses.
- To serve with pasta, you can toss the cooked pasta directly with the pesto. Or, if you want to thin out the pesto sauce a little, add a spoonful or two of the hot pasta water to the pesto, then toss with the cooked pasta and serve right away.