Add the anchovies fillets and let them melt completely and meld with the onions into a kind of creamy sauce.
Meanwhile, bring a large pot of well salted water to the boil, then cook your bigoli until al dente.
Add the bigoli to the onion and anchovy sauce, along with most of the minced parsley if using and a small ladleful of the cooking water. Toss the pasta in the sauce over medium heat until the sauce reduces and clings to the pasta.
Serve right away, topped with the remaining parsley.