1. Place the fish in a deep dish with a spoonful of water. Cover with another plate and place in the microwave, cook at maximum power for approximately one minute.
2. Remove from the microwave and let it cool. With the help of a fork, crumble the cooked fish into small pieces.
3. Wash and cut the tomato in half, remove the seeds, and grate i the pulp.
4. Chop the gherkins.
5. Mix the fish with the mayonnaise, tomato, gherkins, and a few drops of lemon juice.
6. Spread the filling on the sliced bread and serve.