In a stockpot or similar pot, sweat the onions, leeks, and celery in the olive oil over moderate heat until they begin to get tender, 3 to 5 minutes. Add the garlic and sweat until aromatic, about 1 minute. Add the fish heads and bones and continue to sweat gently over low to medium heat for 2 minutes.
Add the tomatoes, orange zest, Pernod, bay leaves, thyme, c