If using a mortar and pestle, add in the garlic and grind to a paste. Then add in the walnuts and crush into a paste as well. Then add in the basil mix to combine. Add in the oil, parmesan and salt and pepper to taste and mix vigorously.
If using a food processor, add in the garlic and basil leaves and process. Then add in the walnuts, parmesan and olive oil and blend again. Add salt and pepper to taste.