Heat oil and butter in a pan.
Add onions and salt. Mix well. Cover and cook till onions is soft on low heat.
Now cook till onions get caramelized. it will take around 15 to 20 mins.
Add little water and scrape the bottom.
Now add in salt, pepper, thyme and mix well.
Add chicken stock and vinegar and mix well.
Bring it to a boil. Cover and simmer for 10 to 15 mins.
For serving. Pour soup into oven safe serving dish.
Top with bread slice and cheese.
Pop this under broiler and bake for 10 mins till cheese is bubbly.