Place the onion slices in a heatproof jar or bowl. In a small saucepan over medium-high heat, combine the vinegar, water, honey, and 1 teaspoon salt. Bring to a simmer for 1 minute.
Pour the vinegar mixture over the onion slices, submerging them completely in the liquid. Cover and refrigerate for at least 1 hour or up to 2 weeks.