Slice one large Vidalia onion into very thin rings or half-moons, separating the pieces as you go. Pack the onion into a clean pint jar or another heatproof container, leaving enough room for the brine.

Add the distilled white vinegar, water, sugar, kosher salt, and any optional peppercorns, garlic, or red-pepper flakes to a small saucepan. Warm the mixture over medium heat for 3-5 minutes, stirring until the sugar and salt dissolve and the brine reaches a bare simmer.

Pour the hot brine carefully over the onions, making sure the slices are submerged. Press the onions down gently with a spoon so the warm liquid can reach every layer.

Let the jar cool uncovered or loosely covered at room temperature for about 30 minutes. The onions may be used at this point, but they will develop better flavor and color after additional chilling.

Cover the jar and refrigerate the onions for at least 30 minutes, or preferably for 4 hours or overnight. Keep them refrigerated and use within approximately 2-3 weeks.