Make the mayonnaise. Whisk egg yolk, dijon mustard and tomato paste. Drain the oil from a 140g can of sardines and slowly drizzle it into the yolk mixture while whisking to build up a nice thick mayonnaise. Season and add a splash of Tabasco to taste.
Put the sardines in a bowl with the onion, capers, parsley and pickle. Gently break up the fish but leave nice chunks. Season. Toast the bread, then heap the sardine mixture on top. Serve the mayo on the side.