Slice the onion and soak in water, then drain. Drain the oil from the canned mackerel and flake roughly with a fork.
You can use the bread as-is or lightly toast it. Spread some mayonnaise over one side of each slice.
Lay the mackerel on the bread and season with salt and black pepper.
Then add the onion. Sandwich 2 slices of bread and press down gently so that the flavors would meld, then cut in half.
So that the mackerel doesn't fall out when eating, press the sandwich down firmly and make sure to take big bites!
Story Behind this Recipe
This is sandwich that my grandfather learnt from an American soldier whilst he was studying in the U.S. According to my grandmother, a young American soldier that they invited over brought this sandwich as a gift. Although that sandwich contained oiled sardines and 1 cm-thick onion slices.
Add plenty of onion, seasoning and mayo for flavor. Apparently, the American soldier that befriended my grandfather used thickly cut onions, but you can cut them to whatever size you like.