To make quick pickled onion, peel a red onion and slice it thinly and place it in a bowl. Pour 1 cup water, ¼ cup apple cider, ¼ cup white vinegar into a sauce pan. Add 1 tbsp sugar and 1 tsp salt to the water and vinegar mixture and bring it to a rolling boil. Simmer for 5 minutes and pour it over the onions. Let it sit for 15-20 minutes and then use the onions in the sandwich.