Caramelise the onions

• Heat the olive oil in a frying pan over medium-low heat.
• Add the sliced red onions with the salt and cook slowly until soft and golden.
• Stir in the brown sugar and balsamic vinegar and cook until sticky and caramelised.

Cool the onions

• Spread the caramelised onions on a plate and let them cool completely.

Make the butter

• Beat the softened butter in a bowl until smooth.
• Fold in the cooled caramelised onions and the black pepper until evenly combined.

Shape and chill

• Shape the butter into a log on a sheet of paper or pack it into a small dish.
• Chill until firm before slicing and serving.