In a medium pot, add water, apple cider vinegar, sugar, salt, coriander seeds and peppercorns. Place over medium-high heat and bring to a simmer, stirring occasionally to make sure the sugar and salt fully dissolve. Remove from heat.
Place onions in a large 16 to 24 ounce glass jar or container. (I used this wide mouth Ball glass mason jarand it fit perfectly.)
Carefully pour the hot vinegar mixture over onions and let sit for 30 minutes at room temperature. Serve immediately or cover and refrigerate for up to 2 weeks.