Pack the sliced onions tightly into a clean pint jar.

Combine the vinegar, water, sugar, salt, bay leaf and peppercorns in a small saucepan and bring to a simmer over medium heat, stirring until the sugar and salt dissolve, about 3 minutes.

Pour the hot brine over the onions, making sure they're fully submerged.

Let the jar cool at room temperature for 1 hour, uncovered.

Cover and refrigerate at least 1 hour before serving, or up to 2 weeks, until the onions turn bright pink all the way through.