Pesto is Italian mole (paste). The primary ingredients are fresh basil leaves, pine nuts, olive oil, and garlic.
It's traditionally made in a mortar and pestle. This is the best way to extract the essential oils of the garlic and blend them properly with the basil leaves. But since I usually prepare larger quantities, I use a food processor.
You can use Parmesan or Romano cheese or both. But if you want to freeze some (for winter when fresh basil is not available or too expensive), leave the cheese out until just before serving. I usually make a double batch and freeze half in an ice cube tray. It's nice to have a taste of summer in the winter.
Some recipes call for the addition of butter for richness. I think it isn't needed if you use a good quality extra-virgin olive oil (don't use any other kind).
If you buy commercial pesto (usually sold in glass jar, sometimes in a toothpaste-tube if imported from Italy), look at the ingredients. Sometimes they corrupt the recipe with parsley, walnuts, xanthum gum, processed cheese, and other fillers. And if you're allergic to nuts, it's possible you're not allergic to pine nuts, which are seeds from pine cones.
The pine nuts are called pignoli in Italy. We don't produce them in the U.S. (I don't know why), s o that's why some recipes here call for walnuts. But imported Italian pignoli are now widely available, and you're more likely to find the cheaper Chinese pine nuts (which are OK).
Pesto makes a great pasta sauce, even better over gnocchi, or in Rick's Pesto Lasagne. The ingredients are very assertive, so you want to use very little (about half as much as a regular tomato sauce), just enough to lightly color the pasta.
Printer-FriendlyPesto
*Makes 2½ cups*
1½ c packed fresh basil leaves
1/3 c olive oil
1/3 c pine nuts
pinch salt
3 large garlic cloves
1/3 c freshly grated Parmesan cheese
Combine all ingredients except cheese in processor or blender and puree until smooth. Can be frozen at this point.
Prior to serving, blend in cheese.