In a heavy-bottomed pan, heat butter and oil. Cook the herbs and chilli flakes for two minutes, or until aromatic.
Add the onions to the pan and cook on a low flame for 15 minutes, until softened and translucent. Increase the flame to medium and cook for another 10-12 minutes until the onions start to turn golden brown.
Add balsamic vinegar, brown sugar, salt, and pepper and stir. On low heat, cook for 10 to 15 minutes, stirring often, until the mixture thickens and becomes dark brown.
Continue to cook until all the water evaporates, and the onions become sticky and jam-like. Remove from heat.
After cooling, store the onion jam in a sterile, clean glass jar.
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