Cut the sardines from the underside to the tail to butterfly them open.
Trim off the tails and fins using scissors.
Press the backbone down with fingertips until the fish flattens and the spine loosens.
Remove the spine and any small bones by hand or with a knife point.
Coat each sardine in salted and peppered flour, then shake off the excess.
Heat 15ml of oil and cook the sliced onion until softened but not colored.
Stir in sugar, vinegar, and capers, then remove from heat and add parsley.
Fry the sardines in the remaining oil for 60 seconds on each side.
Mix the butter with mustard and spread generously over toasted sourdough.
Top the toast with sliced tomato, a sardine, and the warm onion pickle.