Place all of the ingredients in a medium saucepan and bring to a boil over medium-high heat. Reduce the heat to low and simmer until the flavors meld, about 30 minutes. Strain through a fine-mesh strainer into a medium saucepan and keep warm.
For the stew:
Drain the tomatoes and discard the liquid. Using your hands and working over a medium bowl, break the tomatoes into rough 3/4-inch pieces, discarding the cores but keeping any liquid and seeds; set aside.
Heat the olive oil in a large heavy-bottomed saucepan or Dutch oven over medium-high heat until shimmering. Add the leeks and fennel and cook, stirring occasionally, until the leeks are translucent and the fennel has softened, about 5 minutes. Add the garlic, season with salt and pepper, and cook until fragrant, about 1 minute more. Add the wine and let simmer until nearly evaporated, about 3 minutes. Add the reserved tomatoes (along with their seeds and juices) and the warm broth, stir to combine, and bring to a boil.
Reduce the heat to medium low, add the clams, and simmer until they just start to open, about 2 to 3 minutes. Meanwhile, season the fish lightly with salt and pepper. Gently submerge the fish in the broth and bring to a simmer, cooking until the fish just starts to flake when pierced with a fork and the clams open, about 8 to 10 minutes. Taste and season with additional salt and pepper as needed. Sprinkle with the fresh herbs, ladle into bowls, and (if you choose) top each with a dollop of aioli. Serve with the baguette.