Peel onion and cut into thin rings. Turn in flour and fry in hot olive oil until golden brown. Season with salt, drain well on kitchen paper. Fry the bacon in the frying fat until crispy. Drain as well.
Then fry the slices of bread in frying fat until crispy on each side.
Place 1 slice of bacon, 2 sardines and some fried onions on each slice of bread. Drizzle with some pesto if you like.