Trim roots from ends of onion bulbs. Trim the dark green ends to fit the size of a pint-size jar. Slice trimmed onion in half lengthwise. Pack onions in a sterilized, pintsize canning jar, or jar with a tight fitting lid. (Trim more from the green ends, and alternate the direction of the onions as needed to fit them all in the jar.)
In a small pan, combine vinegar, water, sugar, salt, red pepper, and peppercorns. Bring to a boil, stirring to dissolve sugar and salt. Pour brine over spring onions, tuck thyme sprigs into the jar, and let cool. Seal jar and refrigerate pickled onions for at least 24 hours, and up to 3 weeks.