Remove the yolk from the egg halves. Leave one half of the yolk for decoration.
In a blender, chop the mackerel fillets and egg yolks.
Add the softened butter and mustard and whisk again.
For the beauty of the dish, the resulting paste is placed in a pastry syringe or cornet. Squeeze the paste into half of the eggs. Decorate the dish with lettuce leaves, crumbled yolk, dill greens and tomatoes.