Cut the red onion into thin half rings.
Cut the herring fillet into small pieces 1-1.5 cm wide.
Finely chop the green onions.
Take Borodinsky bread. Place pieces of herring on one half of each slice.
Place a little mayonnaise on the other half of the Borodinsky bread.
Place red onion on top of the herring and mayonnaise.
Garnish the sandwiches with green onions. Done. Enjoy!