Make these up to a week ahead, and toss them in your favorite martini for a savory, salty kick. Enjoy these up to a month in the refrigerator.
Combine vinegar, water, sugar, pickling salt, allspice berries, peppercorns, and clove in a nonreactive saucepan; bring to a boil over high. Add onions; reduce heat to low, and simmer 5 minutes.
Transfer onions into 1 (1-quart) canning jar or 2 (1-pint) canning jars, leaving 1/2 inch of room at the top, reserving liquid in saucepan.
Carefully ladle hot vinegar mixture over onions in jars, leaving 1/2 inch of room at the top of each. Discard any remaining liquid. Wipe jar rims. Cover at once with metal lids; screw on bands. Cool jars to room temperature. Onions will keep up to a month in refrigerator.