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1
Slice onions thinly and place in a large, heavy-bottomed pan with 60 ml olive oil and 25 g butter over medium-low heat; cook gently for 12–15 minutes, stirring occasionally, until onions are very soft, translucent, and beginning to caramelize slightly (do not brown aggressively).
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2
Add anchovy fillets (cut into small pieces or left whole, depending on preference) and stir gently into the onion; cook for 2–3 minutes, allowing the anchovies to break down and meld into the sauce, then deglaze the pan with white wine if using, scraping up any fond.
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3
Bring a large pot of salted water to a rolling boil (approximately 4 litres); add bigoli and cook until al dente (approximately 10–12 minutes, or follow package directions for whole wheat pasta).
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4
Reserve 200 ml pasta water, then drain the bigoli into a colander.
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5
Add cooked bigoli to the onion and anchovy pan; toss gently and add remaining 25 g cold butter and 150 ml pasta water; toss continuously over low heat for 1–2 minutes, allowing the starch in the pasta water to emulsify with the oil and create a light, flowing sauce.
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6
Season with fine sea salt and black pepper; add more pasta water if the sauce seems too thick (it should coat the pasta lightly without pooling).
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7
Transfer to a serving bowl or plates, drizzle with remaining 20 ml olive oil, garnish with fresh parsley, and serve immediately.