Peel and thinly slice the red onion into rings or half-moons and place them in a clean 16 oz glass jar or container.
In a small saucepan over medium heat, combine the apple cider vinegar, water, sugar, and salt. Stir and heat just until the sugar and salt are fully dissolved (do not boil).
If using optional spices, add them to the jar with the onions.
Pour the hot vinegar mixture over the sliced onions in the jar, making sure they are fully submerged.
Let the mixture cool to room temperature uncovered, then seal the jar with a lid and refrigerate.
The pickled onions will be ready in about 1 hour but taste best after a few hours or overnight.
Store in the refrigerator for up to 2 weeks.
Prep Time: 10 minutes | Cooking Time: 5 minutes | Total Time: 1 hour 15 minutes (includes minimum marinating time)
Kcal: 15 per serving (approximate) | Servings: 8
