Melt the butter with the olive oil in a large heavy based saucepan. Add the onions, salt and pepper, bay leaves and thyme leaves, stir gently, reduce the heat, cover and cook for 30 minutes, stirring every five minutes. Do not brown the onions.
Remove the lid and cook uncovered for a further 20-30 minutes, stirring frequently until the onions have caramelised.
Stir in the flour and cook over a low heat for a further 3-4 minutes stirring vigorously to remove any lumps.
Stir in the wine and stock. Bring to the boil, reduce the heat and simmer for 20 minutes.
To prepare the croutons: Cut the baguettes into 2.5cm/1in slices and place onto a baking tray. Grill for 2-3 minutes on one side.
Preheat the oven to 180C/35OF/Gas 4.
Rub the top of each toasted baguette slice with the garlic clove. Ladle the soup into individual ovenproof soup bowls, leaving an inch gap around the top of the bowl.
Place two of the bread croutons into each bowl and top with heapfuls of the grated cheese.
Place in the oven at for 10-15 minutes or until the cheese is bubbling and golden brown. Serve immediately.