Drain excess oil from the cans and place the mackerel in a large bowl. Mash the mackerel (as if you were preparing mashed potatoes). Chop onions, break bread into small crumbs and place into the mixture along with the eggs.

Deep fry in hot oil (360°F). Lower frying temperature for larger croquettes, raise temperature for smaller croquettes. Turn over to brown both sides. Remove using a slotted spoon and drain on paper towels or brown paper. Season while still warm.