Pour the pickle juice over the red onion, add a pinch of salt and refrigerate overnight. Whisk the egg in a bowl with a pinch of salt. Pour the milk into another bowl and soak the toast on both sides. Then in the egg so that it completely covers the toast. Melt the butter in a frying pan (20g) and fry the toast on both sides until golden brown. Put the toast into a bowl and top with the drained onion and pickled mushrooms. Put another portion of butter in a clean pan and as soon as it starts frying, fry the herring skin side down for about 1-1,5 minutes. Place the herring skin side up on a sandwich.
Pour water, vinegar and a pinch of salt into a saucepan. When the water starts boiling, stir it with a spoon and pour in the cracked eggs (one at a time). Boil for 2-3 minutes and remove with a slotted spoon.
Place the egg on the sandwich and slice it in half before serving for a "wow" effect.
PS. If you have in your fridge a classic herring in onions, you can use it and omit the frying process.
Recipe by Wit Szychowiak. Check out my fanpage Pan Tasak.