Thinly slice the onion. Add olive oil and butter in a pot, cook until butter melted. Add in onion, cover the lid and cook for 10 minutes. Add in two teaspoons salt and one teaspoon sugar continue to cook for 20-30 minutes, until caramelised. 

Add two teaspoons flour, cook for a minute. Add in apple cider vinegar and beef stock, bring it to boil and simmer for 20 minutes.

Pour the soup in a bowl, put a slice of toasted baguette on top.

Sprinkle cheese then bake in preheated oven at 200C (top shelf), until cheese are melted.