Peel the red onion and slice in half from root to tip. Lay each half flat and slice as thinly as possible into half-moons, about 1/8-inch thick. Pack the sliced onions tightly into a 1-pint mason jar or heat-safe container.

In a small saucepan, combine the apple cider vinegar, water, sugar, salt, and peppercorns. Bring to a simmer over medium heat, stirring occasionally, until the sugar and salt dissolve completely, about 2-3 minutes. Do not boil.

Immediately pour the hot pickling liquid over the onions in the jar, pressing down with a spoon to submerge them completely. The onions will wilt and turn bright pink within minutes. Add the fresh lime juice and stir gently to combine.

Let the jar cool uncovered at room temperature for 30 minutes, then seal and refrigerate. The onions are ready to eat after 30 minutes, but flavor deepens significantly after 24 hours. Store refrigerated for up to 2 weeks.