Directions

1. 1. Melt the butter in a large saucepan or stockpot over medium-high heat.
2. 2. Add the onion and cook, stirring occasionally, until softened and translucent, about 5 minutes.
3. 3. Raise the heat to high and add the fish bones and heads, cook, stirring occasionally, for another 5 minutes or so.
4. 4. Add 6 cups water, bring to a boil, then reduce the heat so the mixture bubbles a bit and simmer for about 30 minutes, skimming any foam from the surface. Strain the mixture.
5. 5. Meanwhile, heat the 3 tablespoons olive oil in a large saucepan over medium-high heat and add the leeks, cook, stirring occasionally, until softened and browned lightly, about 5 minutes.
6. 6. Stir in the chard; add 1 cup of the fish stock, then cover and cook for another 5 minutes.
7. 7. Add about half of the remaining stock and bring to a boil; reduce the heat so that the mixture barely simmers, then add the fish.
8. 8. Poach until just cooked through, 10 minutes (the fish is done when easily pierced with a thin-bladed knife; try not to overcook).
9. 9. Whisk 1/4 cup a'ioli into the remaining stock, then gently stir the mixture into the soup.
10. 10. Season with salt and pepper, garnish with the herb, and serve, passing more aioli at the table.