1. Peel and very thinly slice onions. Combine wine, vinegar, and sugar in a large saucepan or Dutch oven and heat gently until sugar is dissolved, stirring occasionally. Bring just to boil.
2. Add onions; they will fill the pan but will soon wilt. Cook over medium heat, stirring constantly, until onions are transparent and soft enough to cut with spoon, 25 to 30 minutes.
3. Remove from heat. Season to taste with salt and pepper. While still hot, spoon into sterilized jars and seal them. Store in cool place.