Cover the bottom of an oven pan with a thin layer of olive oil and and a layes of sliced potatoes.
Over the bed of potatoes, lay the mackerels which have been cleaned (beheaded and gutted).
Over the mackerels add another layer of sliced potatoes, onions, tomatoes, parsley and a sprinkle of Parmigiano. Salt and pepper to taste. Sprinkle olive oil over the last layer and add two cups of water.
Cover the pan with aluminum foil and bake for 45 minutes at 400F. Remove the foil and bake for an additional 30 minutes.