Peel the brown onions and cut them in half. Slice each onion half into thin, even strips.
Place a large skillet or frying pan over medium heat. Add the olive oil and allow it to heat up for a minute or two until it shimmers.
Carefully add the sliced onions to the heated pan. Stir them gently to coat them evenly with the olive oil. Spread the onions out in an even layer, allowing them to cook evenly.
Reduce the heat to low and let the onions cook slowly. Stir them occasionally to prevent sticking and ensure even caramelisation. This process may take around 30-40 minutes, as slow cooking allows the natural sugars in the onions to caramelise and develop their rich flavour.
After the onions have softened and turned a pale golden colour, sprinkle the sea salt flakes and brown sugar over the onions. The salt helps to draw out moisture from the onions, aiding in the caramelisation process, while the sugar adds sweetness.
Stir the onions well to evenly distribute the salt and sugar. Cook the onions for an additional 10-15 minutes, or until they reach your desired level of caramelisation. Remember to adjust the heat as necessary to prevent burning.
Pour the red wine vinegar over the caramelised onions and stir to combine.
Allow the onions to cook for another 2-3 minutes, stirring occasionally to ensure the vinegar is well incorporated. Taste and adjust the seasoning if needed. Just before you remove the pan from the heat, add silverbeet leaves on top of the onion and replace the lid for 2-3 mins to let them wilt slightly from the heat. Once wilted, set aside.