Place one half of the bread slices on the breadboard and spread them with butter.
Put the salmon slices decoratively on top.
Cut the onion in really thin slices and rinse them in a sieve with very cold water.
Place the onion slices on top of salmon and sprinke with dill.
Spread a thin layer of mustard on the remaining bread slices.
Place the slices, with the mustard down, on top of salmon.
Fry the breads golden brown and garnish with an ample spoonful of double sour cream and roe.
Add some black pepper and garnish with dill.