Slice the onions thinly and evenly. Add them to a large bowl.
In a small saucepan, heat the cider vinegar, water, sugar, salt, oregano, and thyme over medium heat. Bring to a boil, stirring occasionally.
Pour the hot mixture over the onions. Let cool to room temperature, stirring occasionally.
Transfer the mixture to a jar or container with a lid and refrigerate for at least one day before serving. The onions will keep in the refrigerator for up to 2 weeks.