Rinse your basil and pluck off the stems. Pat your basil leaves with a paper towel to absorb excess water, and place the leaves on paper towels to air dry. (See notes.)
Grate your cheeses, if needed (I use my blender.) Combine the Parmesan and Romano in a small bowl.
Peel your garlic and place it in a food processor. Pulse until the garlic is cut into small pieces.
Add your basil leaves to the food processor and pulse until they're broken up. Drizzle in the olive oil while pulsing.
Add your combined cheeses to the food processor and process until a creamy paste is formed. There is no need for salt, since the cheese is salty enough.
Use the pesto as a sauce stirred into pasta or on pizza, chicken or fish. Store the pesto covered, in the refrigerator for two weeks, or freeze for up to one month.
Mom's tips: 1. Genovese basil with corrugated leaves is more flavorful than lemon basil with flat leaves. 2. The basil must be air dried to ensure the leaves are completely dry, in order to achieve the right texture. 3. If you don't have a scale to weigh your basil, you can fill up a 2-cup liquid measuring cup twice with packed basil until it's heaping. 4. Imported Pecorino Romano (made from sheep's milk) is key for flavor. You can buy it at an Italian import store or online(affiliate link).