Plunge the onions in boiling water for one minute to soften their skins. Drain and peel. Place onions in a large bowl. Cover with cold water; add salt and let stand overnight in a bowl.
Rinse the onions with water and dry them with paper towels.
Place pickling spice in a piece of cheesecloth or a coffee filter. Tie securely with cooking twine. Place the vinegar and the spice pouch in a large saucepan. Bring to a boil; reduce heat to medium and cook for 5 minutes. Discard the spice pouch. Let vinegar mixture cool to room temperature.
Pack the onions into clean, hot jars and pour the vinegar mixture over them. If necessary, add additional malt vinegar to cover.
Seal the jars and store them for at least 2 weeks before eating.