- Melt the butter in a medium heavy-based saucepan.
- Add sugar.
- Stir over low heat until dissolved.
- Add onions and cook.
- Cover and continue to cook on low heat for 30 minutes.
- Add garlic and balsamic vinegar.
- Season and cook uncovered on low heat for another 30 minutes.
- Stir every few minutes.
- Once cooked, transfer to a bowl.
- Cool slightly before serving.