Gather the ingredients.
Drain the sun-dried tomatoes, reserving the oil in a liquid measuring cup. Add enough olive oil to the reserved sun-dried tomato oil to make 1/2 cup total.
Add the walnuts to a food processor and pulse a few times until coarsely chopped.
Add the sundried tomatoes, oil mixture, basil, Parmesan, garlic, lemon zest, salt, and red pepper flakes (if using).
Process until the desired consistency is reached, stopping to scrape down the sides once or twice. If needed, add more olive oil in a slow stream (up to 1/4 cup more).
Taste for seasoning and adjust, if necessary, Use immediately or store in the fridge.
