Slightly loosen the sauerkraut with two forks. Peel and finely dice the onion. Mix together with the oil in a baking dish and rest with the lid closed for about 60 minutes.
Slice the small rye rolls horizontally, brush with butter and spread the sauerkraut evenly on top.
Top each with salmon, egg slice caviar substitute and chervil.
Tip: Instead of salmon, you can also use smoked sturgeon, Schillerlocke and trout.