Step 1To make the caramalised onion, heat the oil in a frying pan over medium heat (we used a deep 8-inch/ 20 cm cast iron skillet). Add the onions, and cook, stirring occasionally until the onions start to soften, around 12-15 minutes. Lower heat, add vinegar and salt, and cook, continuing to stir occasionally, until dark and caramelised. Set aside.