Pickled Red Onions

In a small saucepan, combine the apple cider vinegar, water, sugar, salt, peppercorns, and bay leaf. Heat over medium heat, stirring occasionally, until the sugar and salt dissolve completely, about 2–3 minutes.
Place the thinly sliced red onions in a clean, heatproof jar or bowl.
Pour the hot vinegar mixture over the onions, ensuring they are fully submerged.
Let the mixture cool to room temperature, then cover tightly with a lid or plastic wrap.
Refrigerate for at least 1 hour before using, though for best flavor, let them pickle for 24 hours. The onions will keep in the fridge for up to 2 weeks.