Will keep in the refrigerator for 3-6 months at least.
Allowing the onions to marinate in the fridge for a week or so makes they taste very good.

Ingredients

3-4 large red onions
1 cu cider vinegar
1/2 cu red wine vinegar
3/4 cu sugar
1 t. kosher salt
1/8 t allspice

Directions

Slice onions thin. Mix all other ingredients together. Pour over onions - Let stand at room temperature for a few hours. Refrigerate, covered, in a glass container.