Start by peeling your onions, and slicing them as thin as possible. Keep 2 colors of onion separate.
Start heating 2 pans each with 500ml of red wine vinegar & white wine vinegar, then add the red onions to one and the white onions to the other.
Add 2 sprigs of thyme, 2 bay leaves, and 1 tablespoon of black peppercorns to each pan. Simmer for 10 minutes until soft.
Place both colors into a large jar with a lid. Pour the remaining liquid over the onions, then seal the jar and leave to cool.
Refrigerate for up to 2 weeks before serving cold. It is best to eat them within 2 weeks of making them.