Gather all ingredients.

Pack onion slices into a sterilized 1-pint jar.

Combine vinegar, salt, sugar, and peppercorns together in a small saucepan; bring to a boil over medium-high heat.

Carefully pour brine solution over onions to fill the jar; let stand, uncovered, for 30 minutes.

Seal the jar with a tight-fitting lid and shake well. Refrigerate until flavor develops, at least 1 hour or up to 2 weeks, before serving. Keep refrigerated.