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Heat oil in a large heavy pot over medium heat. Cook onion, stirring occasionally, until golden and softened (6-8 minutes). Add garlic and cook (stirring often) until fragrant and garlic is softened, about 2 minutes.
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Add white wine and cook (stirring often) until the smell of the alcohol is almost completely gone, about 4 minutes.
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Add tomato sauce and chopped tomatoes and increase heat to medium-high. Cook, stirring often, until sauce thickens slightly, 8-10 minutes. Taste and season sauce with salt.
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Meanwhile, cook noodles/pasta in a large pot of boiling salted water, stirring occasionally, until very al dente, about 3 minutes less than package directions. Drain, reserving 1 cup noodles cooking liquid.
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Use the noodles cooking liquid to cook the mussels, clams and shrimp, in a large pot. Cover and cook, shaking the pot occasionally, until mussels open, about 4 minutes. Using tongs, pick out shrimp and mussels and transfer to a large bowl, discarding any mussels and clams that have not opened (important!).
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Add the seafood, frozen peas and butter (butter mellows the acid from the fresh onion and the tomatoes) and cook for another 4-5 minutes. Mix in parsley and lemon juice.
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Transfer the noodles into the pot, stir to mix well with the sauce. Top with the chopped parsley, serve immediately.