How to Make It
In a large, deep skillet, heat 3 tablespoons of the olive oil until shimmering. Add the crushed red pepper and cook over high heat for 20 seconds to season the oil. Add the squid and cook, stirring, until opaque, about 1 minute. Using a slotted spoon, transfer the squid to a plate.
Add the mussels, clams and water to the skillet, cover and cook over high heat, stirring occasionally, until the shells open, 2 to 3 minutes for the mussels and 5 to 8 minutes for the clams. Transfer the mussels and clams to a large bowl as they open. Remove the mussels and clams from their shells and rinse briefly to remove any grit. On a work surface, coarsely chop the mussels, clams and squid.
Using a serrated knife, cut the baguette almost in half lengthwise, leaving one side attached. Scoop out the soft, white bread from the center of the baguette and tear it into 1/2-inch pieces.
Wipe out the skillet, add 1 tablespoon of olive oil and heat until shimmering. Add the bread pieces and cook over moderate heat, stirring constantly, until golden and crisp, about 5 minutes. Add the seafood, lemon juice, parsley and the remaining 2 tablespoons of olive oil; season with salt and pepper. Spoon the filling onto the baguette, cut it crosswise into 4 pieces and serve.