Chop the onion first in half and then in long and thin slices. Stir fry them in extra vergin olive oil making sure they don't become golden.
To keep the onions white, cook them on a low heat, with the lid on and, if they become too dry, add a bit of water.
Wash the external salt away from the preserved sardines, take out the central fishbones and then chop them roughly and add them to the onions once the onions are almost cooked.
Stir fry the sardines with the onions for few minutes (the sardines should dissolve in the sauce rather quickly).
Add freshy grated pepper and mix the sauce with the cooked bigoi (simply boil them in water like normal spaghetti) and stir fry everything in the pan for a couple of minutes before serving.