Mix the mayonnaise and crème fraîche together in a small bowl. Add the curry powder and pickling liquid from the red herring. Taste and adjust the seasoning with salt and black pepper or a little more curry powder, if liked. Fold in the herring.
To assemble, spread thinly sliced rye bread with krydderfedt (bacon or pork dripping), then top with the curried herring and a little mustard a