Place the onion along with the vinegar, water, salt, sugar and whole spices and any optional additions in a small pot on the stove and bring to a simmer. Simmer 2-3 minutes, stirring.
Remove form the stove and let cool.
Place onions and liquid in a jar and store in the fridge, for up to 2 months.
Add fresh herbs if you like.
For a Mexican version of pickled onions, add sliced jalapeno and whole cumin and coriander seeds plus Mexican oregano to the pickling liquid. For a Nordic version of pickled red onions, add fresh dill, caraway and peppercorns. For an Indian version of pickled onions add fennel seeds, coriander and cilantro!
optional additions: 1–2 garlic cloves, cut in half a couple bay leaves a teaspoon dried or fresh oregano ( if using on Mexican food)