How to Make It
In a mini food processor, combine the garlic, egg yolk, lemon juice and water. With the machine on, slowly drizzle in 1 cup of the olive oil. Season the aioli with salt.
In a large pot, heat the remaining 2 tablespoons of olive oil. Add the shallot, tomato and saffron and sauté over moderately high heat until fragrant, about 1 minute.
Add the fish stock and Pernod and bring to a simmer.
Add the seafood in stages according to cooking time, starting with the fish that takes longest to cook: Cook the halibut and shrimp until white throughout, flipping once, about 8 minutes; cook the clams and mussels until they open, 3 to 4 minutes; and cook the scallops until just cooked through, about 3 minutes. Discard any clams or mussels that don’t open.
Using a slotted spoon, transfer the seafood to 4 serving bowls. Pour half of the broth into a large bowl and vigorously whisk in 1/2 cup of the aioli until smooth. Pour the aioli-enriched broth back into the pot and whisk once more. Pour the broth over the seafood in the serving bowls; garnish with the tarragon. Pass the remaining aioli at the table.