From My Bowl

Slice the Onion: Slice your onion into half or quarter-sized rings. I prefer to slice mine to be about 1/4″ thick, but that depends on personal preference. Use a mandoline if you’d like your slices to be more uniform and/or thinner. Break up the onion slices and stuff them into a quart-sized glass mason jar.
Pickling Liquid: Add the vinegar, water, salt, sugar, peppercorns, and any additional add-ins (see notes) to a small pot or pan over high heat. Stir the mixture together to dissolve the salt and sugar and bring to a boil.
Pickle: Carefully pour the picking liquid into the glass mason jar and let it sit on your counter until it cools to room temperature. The onion pieces will gradually soften and shrink with time – if any stick above the liquid in the beginning, press them down with a fork 5 to 10 minutes later.
Store & Serve: Seal your jar, then store in the fridge for up to one month. These pickled onions will be the most crispy within the first week, then will gradually soften with time. Serve as desired.