Walnuts and the bitterness trap: walnuts contain tannins and tannic acids that are released more intensely when blended at high speed than with pine nuts. This adds to the olive oil polyphenol dynamic and can make the pesto unpleasantly bitter. The solution has two stages: first, dry-toast the walnuts briefly (1 to 2 minutes over medium heat, until they smell aromatic) – toasting noticeably softens the sharpness of the tannins. Second, keep the steps separate: first coarsely pre-chop the dry ingredients on speed 5 to 6, then stop and let the oil run in slowly on speed 1. Stirring in the last third of the oil by spoon is especially worthwhile with walnut pesto.
Choosing the basil: for this recipe, large-leaf basil works just as well as classic Genovese basil. The stronger aroma holds up against the earthy walnut note. If you want to emphasize the walnut character even more, you can replace 5 g of the Parmesan with a little Pecorino Romano – this strengthens the savory contrast.