For the shrimp stock: In a large saucepan over high heat, heat the oil until almost smoking. Add the shrimp shells and tails, onion, carrot and celery and saute, stirring occasionally, for 5 minutes. Add the wine and cook until reduced by half, then add 10 cups of cold water, the tomatoes, parsley, bay leaf and peppercorns. Bring to a boil, reduce the heat to medium low and simmer, skimming the surface occasionally, for 40 minutes.
Strain the stock through a strainer lined with cheesecloth into a large bowl, pressing on the solids to extract as much liquid as possible; discard the solids. The stock can be made 2 days in advance and stored tightly covered in the refrigerator, or frozen up to 3 months.
For the tomato aioli: Combine the mayonnaise, anchovies, tomato powder, tomato paste, green onion, lemon juice, lemon zest, salt and pepper in a small bowl. Cover and refrigerate for at least 30 minutes to allow the flavors to meld.
For the broth: Heat the oil in a large, high-sided saute pan over high heat and cook the fennel and onion until they begin to soften. Add the pastis and cook until reduced by half, about 5 minutes. Add the tomatoes, saffron, chile flakes and 2 cups of the shrimp stock. Bring to a boil, add the garlic and cook until reduced by half, about 10 minutes. Add 4 more cups of shrimp stock, reduce the heat and cook until the flavors meld and the broth reduces slightly, about 20 minutes. Strain the shrimp-tomato broth into a large bowl, discard the solids, pour the liquid back into the high-sided saute pan and set aside.
For the anchovy butter: Put the anchovies and butter in a bowl and mix until combined. Season with salt and pepper, cover and refrigerate until cold, at least 20 minutes.
For the seafood: Heat 2 tablespoons of the oil in a large saute pan over high heat. Season the scallops on both sides with salt and pepper. Sear the scallops on one side until golden brown, about 2 minutes, then turn over and cook for 30 seconds. Remove to a baking sheet.
Add another tablespoon of the oil to the pan and season both sides of the halibut fillets with salt and pepper. Sear on one side until golden brown, then turn over and cook for 30 seconds. Remove to the baking sheet with the scallops.
Heat the remaining tablespoon of oil in the large saute pan, season the shrimp with salt and pepper and sear until the shells become lightly golden brown, about 1 1/2 minutes per side. Deglaze the pan with 1 cup of the reserved shrimp-tomato broth. Transfer the liquid to the remaining shrimp-tomato broth in the high-sided saute pan.
Heat the shrimp-tomato broth over high heat. Add the mussels, cover and cook until they open, about 2 minutes. Remove the mussels to the baking sheet with a slotted spoon and discard any that did not open.
Add the seared seafood and lobster tails to the shrimp-tomato broth, cover and let cook for 2 minutes.
To serve: Add the anchovy butter to the broth and cook until thickened; stir in the parsley, tarragon and lemon zest. Top each baguette slice with a large dollop of tomato aioli and garnish with chives. Divide the seafood and broth among 6 large, slightly shallow soup bowls. Top each with a tomato aioli crouton and garnish with more parsley or tarragon or chives, if desired.