Add the vinegar, water, olive oil, garlic, honey, oregano and salt to a pan. Bring to a simmer, remove from the heat.
Add the rosemary sprigs to 2-16 ounce (500 ml) sterilized jars (see notes how to sterilize jars). Fill with the sliced onions. Pour over the hot vinegar mix and seal and allow to cool.
Once cooled, refrigerate for up to 2 weeks.
Video
Notes
**To sterilize the jars, wash with warm soapy water and rinse well. Arrange the jars and lids onto a baking sheet and leave in a preheated, 175 degree F/80 degree C oven for 25 minutes. Or, boil the jars and lids in large saucepan of boiling water for 15 minutes. Allow to dry completely.