Fry the onions in the butter until they turn golden and then add the mushrooms, garlic and the leek and fry off for 5 minutes, stirring every minute or so.
Add the stock and the thyme, bring to the boil and then reduce and simmer for ten minutes
While that is simmering away, gently fry the red onion in the oil and the butter and when it starts to loosen add the balsamic and the sugar and simmer for 10 minutes until soft and sticky
After the soup has been simmering for 10 minutes, remove from the heat and blend into a smooth and glossy consistency and return it to a medium – low heat
Make the milk and the flour into a smooth paste and stir it into the soup, continuing to stir until the soup thickens slightly.
Lightly toast the bread and cut it into squares. Top with the onions and then crumble with the cheese – place under the grill for 2 minutes or so until the cheese has started to melt.
Season the soup to taste and serve hot, topped with the toasts and fresh parsley.