S.O.S.

Instructions

1. In a bowl, whisk the mustard, vinegar and sugar until fully dissolved.
2. Add the mayonnaise and ketchup, then mix until you have a smooth sauce.
3. Loosen with the hot water a spoonful at a time to get a runny texture, then season with salt and pepper.
4. Cut the herring fillets into pieces and slice the onion into thin rings.
5. Arrange the herring pieces and onion on the rye bread, then drizzle with the S.O.S. sauce.