Clean the mackerel and remove all the bones. Chop the onion. Remove all bones from the smoked fish carcass. Chop the gherkins and capers as finely as possible. Peel the cucumber and cut into small cubes.
Preparing the sauce: Break an egg into a blender bowl, add mustard, salt, sugar, vinegar and 100 ml of vegetable oil. Beat until smooth, add another 100 ml. vegetable oil and beat again.
Add gherkins and capers to the sauce. Cut frozen and hot smoked mackerel fillets into small cubes and chopped fresh cucumber, add salt and pepper, add sauce and mix everything together.
Place the fish tartare in the center of the dish, and arrange chopped green onions, cucumbers, gherkins and capers around it, serve the sauce in a separate bowl with pieces of fragrant Borodino bread.