Step 1

If using clams, while you prep the rest of the soup, soak them in a deep bowl of cold water, so they can discharge any grit.

Step 2

In a large lidded pot or Dutch oven over medium heat, heat the oil and melt the butter. Add the leeks, onion, fennel bulb, if using, garlic and salt. Cook, stirring occasionally, until the onion and leeks soften and turn translucent, 6 to 8 minutes.

Step 3

Increase the heat to high, and stir in the wine. Bring to a boil and cook until reduced by half, about 2 minutes.

Step 4

Add the potatoes, if using, water, tomatoes and saffron, crushing it between your fingers as you sprinkle it into the pot. Give everything a good stir, cover the pot and return the liquid to a boil. Leave the lid very slightly ajar and cook, stirring occasionally to ensure nothing is sticking to the bottom of the pot, until the potatoes are easily pierced with a fork, 15 to 18 minutes.

Step 5

Reduce the heat to medium. If you have added the potatoes, they will have absorbed a lot of the seasoning, so taste the broth, and season with more salt, if desired. If using clams, drain, and discard any that do not close when gently tapped. Add the shellfish and/or fish and, if they’re not submerged in the liquid, add additional water to cover by 1/2 inch. If using only fish, simmer uncovered until it’s cooked through, 1 to 3 minutes, depending on the type of fish. If also — or only — using shellfish, cover the pot and cook until the mussels or clams open, 2 to 3 minutes. (Discard any mussels or clams that do not open.)

Step 6

Remove from the heat. Taste the broth, and season with more salt, if desired, and a few turns of freshly ground black pepper. Add 2 tablespoons of the parsley, and very gently stir to combine, taking care not to break up the cooked fish. Divide among individual bowls, garnish each with a few fennel fronds, if using, and the remaining parsley, and serve, with the toasted bread slices for dipping into the broth.