Combine yoghurt, capers, gherkins and dill, stir well then set aside. Thinly slice salmon.
Brush one side of each slice of bread with a bit of olive oil. Toast oiled side on a hot grill pan or skillet.
Set out 4 slices of bread, toasted side down. Layer with greens, sauerkraut and salmon. Top with yoghurt sauce, sliced onions and remaining slices of toasted bread.
Cut in half on the diagonal, because sandwiches always taste better in triangles, and serve immediately.
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