This is a sauce to accompany Mediterranean fish soups. You can stir it into the broth or dab it on the fish. Some call for saffron and sweet red pepper, some not. All call for chili or Cayenne pepper. That said, try using harissa. This recipe is an unorthodox amalgam of others that produces a well-flavored emulsion.
It takes patience, plus a marble mortar and pestle large enough to accommodate the ingredients to make the pesto. The result is so much better than the one done with an electric appliance. If you resort to the blender or food processor, use short pulses to achieve a fairly rough texture.