Step 1

Fill a large bowl with cold water and stir in enough salt to make it salty like the sea (a 3% salt solution, by weight, is a good ratio). Add clams and let stand for 30 minutes. Lift clams from bowl and discard sandy water. Rinse out bowl and repeat this purging process until clams no longer release sand into the water (usually 2 to 3 purging cycles). Discard any clams that are gaping open and refuse to close when tapped. Set aside.

Step 2

Heat 3 tablespoons (45ml) oil in a 12-inch skillet over medium-high heat until shimmering. Add shallots, season lightly with salt, and cook, stirring occasionally, until shallots start to soften, about 5 minutes. Add garlic and red pepper flakes and cook, stirring constantly, until fragrant, about 30 seconds.

Step 3

Add wine and clams, increase heat to high, cover, and cook, shaking pan frequently, for 4 minutes. Uncover and, using tongs, transfer any clams that have opened to a medium bowl; cover bowl with foil to keep warm. Add mussels to skillet, cover, and continue to cook, shaking pan frequently and uncovering to stir and check shellfish every 45 seconds, until clams and mussels have all opened, 2 to 3 minutes longer. Using tongs, quickly transfer shellfish to bowl, and cover; leave aromatics and liquid in pan.

Step 4

Add clam juice and 2 cups (475ml) water to skillet and bring mixture to a boil over high heat. Add spaghetti and cook, stirring constantly for the first 30 seconds to prevent noodles from sticking and then stirring occasionally, until pasta begins to have some flexibility and is softened on the exterior, but well shy of al dente and still uncooked in the center, about 3 minutes less than the package directions. Add water in 1/4 cup (60ml) increments as needed to ensure pasta cooks evenly and doesn't stick to the bottom of the pan.

Step 5

Meanwhile, pull meat from about half of the clam shells and half of the mussel shells; discard empty shells and return shucked clams and mussels to bowl with remaining shell-on shellfish. Cover to keep warm.

Step 6

When pasta is just softened on the exterior, add shrimp, squid, and tomatoes to skillet. Continue to cook, stirring and tossing, until shrimp and squid are just cooked through, 2 to 4 minutes.

Step 7

Add all of the mussels and clams, along with any accumulated juices, and continue to cook, stirring and tossing constantly, until all of the seafood is heated through and pasta is al dente, about 1 minute; add more water, 2 tablespoons (30ml) at a time, if sauce over-reduces and becomes dry or greasy. Remove from heat, add parsley and lemon juice, and toss rapidly to incorporate. Season with salt to taste.

Step 8

Transfer to a large serving bowl or divide between individual serving bowls, arranging shell-on mussels and clams on top of pasta. Drizzle with remaining 1 tablespoon (15ml) oil, and serve.