From the Moosewood Cookbook:
pickled red onions
1 cup cider vinegar
1 cup water
up to 3 tbsps brown sugar
1 tsp salt
1 tsp whole peppercorns
4 medium red onions, very thinly sliced
combine vinegar, 1 cup water, sugar, salt and peppercorns in a medium sized bowl, and stir till the sugar is dissolved.
place the onion slices in a colandar in the sink, and slowly pour a kettle-full of boiling water over them; they will wilt slightly (well, so would you ...). Drain well, and transfer to the bowlful of marinade.
cover, and allow to marinate - at room temperature, or in the fridge - for several hours. Store in the fridge, and use as needed.
(I like to put them on burgers or anywhere else you'd used a bread & butter pickle)