Remove basil leaves from stems and discard any damaged leaves.
Rinse leaves and remove excess water. I like to use my salad spinner for this, but you can also press the leaves between paper towels to remove excess moisture.
Add 2 cups loosely packed basil leaves to a food processor bowl.
Add garlic clove. I like to rough chop before adding to the food processor.
Add 1/4 cup toasted walnuts. For toasting walnuts, spread a single layer on a parchment paper lined baking sheet. Bake at 350 degrees for about 10 minutes, turning occasionally. I like to rough chop before adding to the basil mixture.
Add 1/4 teaspoon salt, and 1/4 teaspoon fresh ground black pepper.
Add 3 ounces good quality parmesan.
Process about 15 seconds, then turn off the machine and scape down sides of bowl and process another 10-15 seconds until all ingredients are broken down.
While running the processor, slowly add 1/2 cup olive oil and continue running the processor until you reach the desired consistency. You can add more olive oil if you prefer a thinner pesto.