Herring clear from the skin and viscera, cut off the head, remove from range, select all the bones, cut into slices and put in any container You like. I put in a container with a wide bottom, but you can in the jar.

Onions cleaned, cut into half rings and pour the water, once the bitterness is gone.

In a bowl or mug, put two teaspoons of mustard (I put right with a slide, as spacecolony) and dissolve mustard lukewarm water. Add the water gradually, so that lumps are not formed. Sprinkle with sugar, add vinegar and sunflower oil (who likes, and smell). Stir everything so that the sugar is dissolved and try. Basically, I do everything by eye, so that here the proportion is a relative thing... If you pass the mustard, will have to top up with water if not to report, the sauce will be "empty". Well, the vinegar and sugar according to Your taste, sweeter loves who, and who is not...

Water with onion drain and put the onion to the herring. I do a lot of onions, so I have a lot of sauce.

Pour in the mustard sauce. You can eat immediately. You can make not one but, say, two herring, and closing the lid, put into the refrigerator. Not getting worse, on the contrary, all impregnated with the filling and eaten with enthusiasm.

It is possible and so.