1. Bring a large pot of lightly salted water to a boil over high heat. Add the spaghetti and cook according to the package instructions until al dente.

2. While the water is coming to a boil, prepare the sauce. Heat the oil in a large saucepan over medium heat. Add the onion and garlic and cook, stirring often, for 3–4 minutes, or until softened. Add the crushed tomatoes and hot pepper. Simmer for 1 minute. Keep warm.

3. Meanwhile, combine the mussels, wine, and lemon in a large saucepan. Cover and bring to a boil over high heat. Cook for 3–4 minutes, shaking the pan occasionally, until the mussels have opened. Transfer the seafood to a bowl, discarding any unopened shells. Strain the liquid through a fine sieve into the tomato sauce, discarding the lemons. Reserve a few mussels in their shells for garnish and remove the flesh from the remaining shells.

4. Simmer the sauce for 2–3 minutes, or until slightly reduced. Add the shrimp and squid to the sauce and simmer for 2 minutes, stirring, until just opaque. Add the reserved mussels to the sauce, stir in the parsley, and season with salt and pepper.

5. Drain the pasta thoroughly and return to the pan. Add the seafood sauce and toss. Transfer to deep bowls, topping each portion with a few mussels, and serve hot.