Sauté onion in butter and oil until tender and golden. Add flour and cook stirring for 2 minutes. Add wine and beef stock and stir. Pour water and add bouquet garni. Season with salt and pepper and bring to a boil. Cover pan, reduce heat and simmer for 30 minutes. Sprinkle half of cheese, stir and correct seasoning.
Preheat oven at 400 °F (200 °C). Place baguette slices on oven rack and toast until golden.
Ladle the hot soup into heat proof soup bowls. Place baguette toast on top and cover with rest of cheese.
Preheat broiler and broil soup until cheese melts. Serve very hot.