Step 1

Pick any fronds from 1 medium fennel bulb and reserve for garnish. Core and finely chop the bulb (about 1 1/2 cups).

Step 2

Heat 3 tablespoons olive oil in a 5 1/2-quart or larger Dutch oven or heavy-bottomed pot over medium heat until shimmering. Add the fennel and cook until starting to soften, about 2 minutes. Stir in 2 finely chopped shallots and 1/2 teaspoon of the kosher salt, and cook until the vegetables are tender and translucent, 3 to 4 minutes more.

Step 3

Stir in 6 thinly sliced garlic cloves and 1/4 teaspoon red pepper flakes, and cook until fragrant, about 1 minute. Add 2 tablespoons tomato paste and cook, stirring often, until the paste darkens in color, about 2 minutes.

Step 4

Stir in 1 cup dry white wine and simmer until the wine is reduced by about half, about 2 minutes. Stir in 1 quart fish or seafood stock, 1 (28-ounce) can diced tomatoes and their juices, and the remaining 1 1/2 teaspoons kosher salt. Increase the heat to high and bring to a boil. Reduce the heat to maintain a very lively simmer and cook for 30 minutes to reduce slightly and let the flavors meld. (This is a good time to prepare the seafood.)

Step 5

Stir in 1 pound mussels and 1 pound clams. Cover and cook until most of the mussels and clams begin to open, 3 to 4 minutes. Gently stir in 1 1/2 pounds cut white fish fillets and 1 pound raw shrimp, making sure they are mostly submerged in liquid. Return to a simmer.

Step 6

Gently stir again to move the fish and shrimp at the top closer to the bottom. Remove the pot from the heat. Cover and let sit for 2 minutes to let the residual heat gently finish cooking the seafood (the fish and shrimp should be opaque).

Step 7

Taste the broth and season with kosher salt as needed. Garnish with 2 tablespoons coarsely chopped fresh parsley leaves and the fennel fronds. Ladle into serving bowls, discarding any clams or mussels that have not opened. Serve with toasted or grilled sourdough on the side.