Onion peel and in rough cubes cut. Blanch for 3-4 minutes in boiling water. Drain and let cool. Peel and quarter apples and cut out the core. Cut apples into pieces.
Wash the dill, chop finely and mix about half with the onions and apples. Season vinegar with salt, pepper and sugar. Beat 5 tablespoons of oil into it. Pour over the apple-onion vegetables and marinate for about 30 minutes.
Peel and wash the potatoes. Cook in salt water for about 20 minutes. Wash mackerels, dab dry and season inside and out with salt and pepper. Dust all around with flour. Heat the remaining oil in a pan and fry the mackerels over medium heat for 8-10 minutes on each side.
Arrange on a plate together with the apple and onion vegetables and garnish as desired with apple slices, slices, dill and blossoms. Drain the potatoes, sprinkle with the remaining dill and serve.