Sterilize jars and lids (boil 10 minutes or run through a hot dishwasher cycle)
Choose onions (red, yellow, or shallots); peel and trim
Cut onions (whole, halves, or slices); keep pieces consistent
Optional: Blanch onions 30–60 seconds for milder flavor; drain and cool
Pack onions tightly into clean jars
Add flavorings as desired (garlic slices, peppercorns, mustard seeds, dill, chili flakes, bay leaf, thyme)
Make pickling liquid: combine equal parts vinegar and water (or use 100% vinegar)
Add salt to the liquid (about 1 tablespoon kosher salt per 1 cup liquid); avoid iodized salt if possible
Optional: Add sugar (about 1–2 teaspoons per cup liquid) for mild sweetness
Bring pickling liquid to a boil, stirring to dissolve salt (and sugar if using)
Pour hot brine over onions, leaving about 1/2 inch headspace
Remove air bubbles with a nonmetal utensil; re-check headspace
Wipe jar rims clean; apply lids and rings
Process for shelf-stable results (water bath): 10 minutes for half-pints/pints (adjust for altitude if needed)
Cool jars undisturbed 12–24 hours
Refrigerate and use within 2–3 weeks, or store processed jars in a cool, dark place (typically up to 1 year)
Let onions pickle at least 24–48 hours before eating; best flavor after 1–2 weeks