Heat the butter in a large saucepan over a medium heat, when the butter has melted add the onions, lower the heat and cook the onions for around 8mins until they begin to soften.
Add the sugar and cook for another 5mins until the onions are caramelised. Then add the garlic and stir for another minute.
Gradually add the wine and reduce by half.
Then add the beef stock, thyme, bay leaf and yeast extract and bring to a boil
Then add the cornflour and stir until all combined, cover and simmer for around 10-15mins.
In the meantime, make the croutons. Preheat the grill and toast the slices on one side.
On the uncooked side spread some mustard very thinly, sprinkle with the cheese and pop back under the grill until the cheese has melted.
Divide the soup with into bowls, serve immediately with the croutons on the side.
