Add the apple cider vinegar, water, salt, and sugar to a saucepan. Heat until the salt and sugar have completely dissolved.
Thinly slice the white onion with a chef's knife or a mandoline.
Add the sliced onions to a clean 16-oz (500 ml) glass jar. Pour the pickling liquid on top and add in your spices (black pepper, cardamom, and cinnamon).
Make sure the onions are completely submerged in liquid and close the jar with a lid. Let it cool for at least 20 minutes or until it reaches room temperature.
Place it in the refrigerator overnight or for at least 20 minutes before serving.
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Notes
You can store the quick pickled onions in the refrigerator for up to2 weeks. Make sure they are completely covered in vinegar brine the whole time and always take out the onion whit a clean fork.
If they feel slimy or if some onions were not covered with pickling liquid, it's best to throw them out.