Heat the oil in a lidded frying pan over a low heat and add the red onions and thyme. Cover and cook for 15-20 mins, stirring occasionally so they don’t catch, until soft and slightly golden.

Remove the lid, reduce the heat to low and stir through the sugar, vinegar and 2 tbsp water. Cook for a further 5 mins until the liquid has almost disappeared, then allow to cool.

Mix the butter with the onions until fully incorporated.