Clean the fish, remove the entrails, head, tail, fins and spine. Rinse well with cold water. Peel the onion and bell pepper, wash all vegetables and herbs thoroughly with cold running water.
Salt and pepper the prepared mackerel. Pour cold water over the gelatin and leave to swell. Check the packaging for the method of gelatin preparation.
Cut the prepared onion into small cubes, cut the pepper into thin strips. Add the egg and prepared gelatin to the finely chopped spinach and mix well. Add chopped onion and pepper.
Spread the prepared mixture in a thin layer on the marinated mackerel fillet. Roll into a roll and tie with thread or twine. Wrap in cling film and place in a pan of boiling salted water. After boiling, cook for 20 minutes. Cool and place in the refrigerator until the gelatin hardens. The time depends on the type of gelatin.
Serve the stuffed mackerel chilled as a cold appetizer or as a main course on its own. When serving portionwise, cut into medium-thick slices, place on a plate and garnish with slices of pickles.