Hot-Pink Pearl Onion Pickles

Bring vinegar, sugar, coriander seeds, tarragon, salt, and mint to a boil in a small saucepan, stirring to dissolve sugar and salt. Reduce heat to a simmer and add onions and beet. Cook, stirring occasionally, until onions soften slightly, about 3 minutes. Pluck out and discard beet (the onions will continue to take on color as they sit). Transfer onions and brine to a 1-qt. jar or a nonreactive container and let cool. Cover and chill at least 2 hours before serving. The flavor will mellow as it sits.