Peel and thinly slice the red onion into rings or half-moons, depending on your preference.

In a small saucepan, combine the apple cider vinegar, water, sugar, and salt. Heat over medium heat and stir until the sugar and salt dissolve, about 1-2 minutes. Remove from heat.

Place the sliced onions in a clean glass jar or container.

Carefully pour the warm vinegar mixture over the onions, ensuring they are fully submerged.

Let the onions sit at room temperature for 30 minutes to pickle. For a stronger flavor, refrigerate for at least 1 hour before serving.

Store the pickled onions in the refrigerator for up to 2 weeks.