Toast cumin and coriander in dry pan until fragrant. Remove and set aside.
Add vinegar water, sugar and salt to the same pan. Bring to boil. Stir to combine.
Slice onions, squeeze as many as you can into pint jars. (Depends on how big your onion is.)
Crush garlic. Add to jar. Add spices and garlic to each jar.
Cover with vinegar mixture. Seal jars.
Allow to cool to room temperature, then refrigerate for at least 3 days, up to about 6 months.
**Notes**Best if used with 1 month. They are still edible afterwards, but retain their color best early.Do in small batches.Cut onions very thinly with a chef's knife, or carefully on a mandolin.Can substitute vidalia onion, if you prefer a milder flavor.