Peel the onions, cut them in half and slice very thinly, pour the oil in a large pan, add onions and sauté over low heat without letting them colour, possibly adding a little hot water if they begin to dry out.
Meanwhile, rinse the anchovies to desalt them, remove the backbone and any other bones, cut into pieces.
When the onions become transparent, add the anchovies and let them melt slowly and completely. Then add pepper to taste.
Cook the Bigoli in abundant salted water. Drain when al dente (but keep a few tablespoons of cooking water!), set the pasta aside.
Dress the Bigoli with the hot sauce of anchovies and onions, then serve.