Herring, Onion, Vinegar
Slices of herring (clean of bones, heads and other stuff) place into long medium deep plate.
Put rounds of onion on top to cover all fish.
Add veg oil (can be any of your choice, if choosing from equal ones pick that with a stronger smell)
And a tsp or two of vinegar, depending on your taste. Do not forget to mix it with water as due to concentration, indicated on the bottle.
Before serving let it rest to have all liquids and juices mix together. Some use a fridge where for curing, we use room temperature for half an hour or an hour, it’s your food after all and totally your decision, hehehe.