Add the garlic to the hot walnuts which will help to cook the garlic slightly.
Add the remaining ingredients and blend to preferred constituency; nasturtium leaves, parmesan, extra virgin olive oil, lemon juice and salt and pepper to taste. As the leaves are already peppery, taste test before adding pepper.
Spoon pesto into a clean jar and store in the fridge for up to 10 days. You can also freeze the leftovers.