Get 2 pans and add 1 tablespoon of sea salt to each. Add the red wine vinegar and thyme to one pan and the white wine vinegar and bay to the other.
Keeping the colours separate, peel and slice the onions using a food processor, or you can slice them finely with a knife.
Place both pans over a high heat and bring to the boil. Place the red onions in the pan with red wine vinegar and the white onions in the other pan.
Reduce the heat to medium and cook for 2 to 3 minutes, pushing the onions down with the back of a wooden spoon to stop them rising to the surface.
As soon as the onions have softened, use a slotted spoon to transfer them into sterilised jars. Ladle over the vinegar, then close the lids and leave to cool. These can be eaten straight away, but if you leave them for at least 2 weeks they’ll taste amazing.