Meanwhile, combine the vinegars, syrup and salt in a glass measuring cup. Stir to combine and dissolve the salt. Add the remaining seasonings (peppercorns, allspice berries, rosemary, and garlic) Set aside.
Set a small strainer inside a medium bowl. Place the onions in the strainer. Pour the boiling water over the onions. Carefully (using an oven mitt if needed) lift the strainer out of the bowl. Discard the hot water.
Add the onions to your desired glass container. I used a medium sized glass Weck canning jar. Pour the vinegar mixture over top of the onions, gently mixing to distribute the flavors.
Store in the refrigerator for at least 30 minutes before using. Keep refrigerated in between uses. Onions will keep for approximately 2 weeks.