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Heat the oil in a large non-stick pan and cook the fennel and onion for 10 mins, stirring occasionally until softened. Meanwhile, bring a pan of water to the boil and cook the spaghetti for 10 mins until al dente.
When the fennel and onion mixture has softened, stir in the chilli flakes, garlic, lemon zest and juice. Add the tomato purée and cook briefly, stirring all the time.
Drain the pasta, reserving a mugful (around 250ml) of the cooking water. Stir the seafood into the tomato sauce and warm through. Toss the spaghetti into the sauce with the parsley and just enough of the reserved cooking water to loosen. Season and serve.