I don't really follow a recipe as it differs everytime usually when I realise I don't have one of the ingredients to hand!
This my baseline I suppose and tweak according to taste.
75g butter
1kg red onions, thinly sliced
1 teaspoon salt
150g light soft brown sugar
375ml red wine
50ml tablespoons red wine vinegar
50ml tablespoons balsamic vinegar
1 tsp Fresh thyme leaves
2 Bay leaves
Melt the butter in a deep saucepan over medium heat; add onions, sugar and salt. Cook slowly over a low heat, stirring every so often, until the onions have softened and most of the juices have evaporated, about 40 minutes. Do not rush this stage, it is crucial to the final product.
Add red wine, red wine vinegar, balsamic vinegar, thyme and bay leaves. Stir to mix, then continue to cook for another 30 to 40 minutes or until the onions have caramelised and the sauce is syrupy.
Remove from the heat. When it has cooled a little remove bay leaves and spoon into sterilised jars and seal with a lid.
Best left for a couple of weeks to mature but can be eaten immediately if impatient.