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Boil the spaghetti for 10 mins until al dente, adding the spinach for the last 2 mins. Meanwhile, fry the onion in the oil in a large non-stick frying pan with a lid until tender and turning golden. Stir in the chilli and garlic, then add the tomatoes.
Add the vinegar, capers, olives and paprika with a ladleful of the pasta water. Put the cod fillets on top, then cover the pan and cook for 5-7 mins until the fish just flakes. Drain the pasta and wilted spinach and pile on to plates, then top with the fish and sauce. Sprinkle over some parsley to serve.