In a small saucepan, combine apple cider vinegar, sugar, and salt. Heat over medium heat until the sugar and salt are fully dissolved.
Place the thinly sliced red onions in a clean jar or container.
Pour the hot vinegar mixture over the sliced onions, ensuring they are fully submerged.
Let the pickled onions cool to room temperature, then cover the jar and refrigerate for at least 1 hour before serving.
Enjoy your homemade sweet pickled red onions on sandwiches, salads, tacos, and more!