In a wide skillet or braiser, sauté sliced red onions over low heat in a generous amount of olive oil, drizzling them from time to time with a few drops of white wine to help soften the onions and prevent them browning.

When the onions have softened and become translucent, add the anchovy filets and the parsley and cook for a few more minutes, just until the anchovies have melted into the oil. Add one last drizzle of wine and let it mostly (but not completely) evaporate. Season to taste with pepper and if you think it needs it, salt. (You should need very little if any.) 

Cook the spaghetti al dente and, when they're done, drain (but not too well, you'll want some of the water to cling to the pasta to help moisten the dish) and turn them into the skillet with the onion and anchovy sauce, along with a few drops of the colatura if using. Mix well, then add the breadcrumbs if using and mix again. (Also add some of the pasta water if you find the mixture too dry; the pasta should slither nicely as you toss it.)

Serve immediately, with some more chopped parsley and a drizzle of olive oil on top.