TO PREPARE AIOLI:
In a mortar with a pestle, grind the chillies to a powder. Add the garlic and salt and crush and pound until a paste forms. Add the breadcrumbs and the saffron and its soaking water and incorporate into the paste. Scrape the paste into a bowl. Add the egg yolks and whisk until the mixture has thickened. Whisking constantly, slowly add the olive oil, a drop at a time, whisking until he mixture emulsifies and forms a mayonnaise-like consistency. Add only as much of the oil as needed to achieve a good consistency. Cover and refrigerate until serving.
TO PREPARE BROTH:
Heat the olive oil over medium-high heat in a large Dutch oven or other heavy-bottomed pot. When it is hot, add the shrimp shells and sauté until fragrant, about 2 minutes. Add the onion and sauce until translucent, about 2 minutes; add the garlic, carrot, celery, fennel, bouquet garni, clam juice, tomatoes and their juices, the water and the salt and pepper. Reduce heat to low, cover and simmer until the flavors are blended, about 15 minutes.
Taste and adjust seasonings as desired.
Using a Chinoise or a heavy sieve lined with several layers of cheese cloth, strain the broth. You should have about 6 to 7 cups of broth.
To finish stew
Place the strained broth in a clean, large pot. Bring it to a simmer over medium-high heat and add the potatoes and the celery. Reduce heat to low and simmer until the potatoes are just tender when pierced with a knife, 10 to 15 minutes. Add the fish, parsley and saffron and cook just until the fish is opaque and easily flakes with a fork, about 4 to 5 minutes. Stir in the clams.
Ladle the soup into bowls and top with a slice of toasted baguette topped with a tablespoon of the aioli. Serve the remaining aioli alongside, with additional toasts if desired.