These are really "faux" potpies, since the pastries are baked separately, but they are elegant nevertheless. Cod or halibut may be used instead of the wild striped bass.
Combine shallot, wine, salt, and 1/2 cup water in a medium saucepan, and bring to a simmer. Add mussels, and cook, covered, until shells are open, about 2 minutes. Remove mussels from shells, and set aside, loosely covered. Discard shells. Strain cooking liquid through a double layer of damp cheesecloth, and set aside.
Heat oil in a medium straight-sided saute pan over medium heat. Add garlic and onion, and cook, until soft, stirring occasionally, about 5 minutes. Add anchovies; cook another 2 minutes, stirring. Raise heat to medium high; add tomatoes, breaking them up slightly with the back of a spoon. Add reserved mussel liquid and red-pepper flakes, and cook until sauce has thickened, about 25 minutes. Transfer sauce to the bowl of a food processor or blender, and puree until smooth.
Heat oven to 425 degrees. Place six 6-ounce ramekins on a baking sheet, and set aside. In a small bowl, combine egg and milk, and set aside. Lay puff pastry on a clean surface. Using a round cookie cutter or the rim of a glass, cut out six pastry rounds a bit larger than the rims of the ramekins. Brush pastry with egg wash; place on a parchment-lined baking sheet. Bake until golden brown and puffed, 9 to 12 minutes. Set aside in a warm place.
Return sauce to saute pan; stir in yellow peppers, thyme, and parsley. Season with salt and pepper. Stir in the bass, calamari, shrimp, and scallops; distribute mixture evenly among ramekins. Place on a baking sheet; bake until juicy and bubbling, 15 to 20 minutes. Place a round of puff pastry on top of each cup, and serve immediately.