In a small jar, mix the vinegar, water, salt and sugar.
Add the onion, pepper corns and bay leaves to the jar and close it.
Shake the jar well to mix the ingredients. Open the lid and make sure that the onion is entirely covered with brine. If necessary, push it down a bit.
Put the jar in the fridge (make sure it is closed firmly) and let it sit. Preferably, it should rest at least one day, but it can be used after 4-5 hours.
Notes
Feel free to play with the ratio of water and vinegar. This recipe is to the milder side, so if you prefer a bit more sour pickles, add more vinegar.
You can add some thin slices of bell peppers, carrots, or cucumber.