Preheat the broiler. Saute the onions in the oil over medium heat until tender and caramelized, 45 minutes. Add the garlic and Swiss chard and saute until wilted, 4 minutes, then add the thyme, salt, pepper and bay leaf. Sprinkle over the flour and cook for a few minutes more, browning the flour well. Add the broth and wine and bring to a boil. Reduce the heat, cover, and simmer gently for about 20 minutes.
Meanwhile, spray the baguette slices with nonstick cooking spray. Toast the slices on a griddle until golden brown. Mix together the cheeses.
Ladle the soup into 4 ovenproof bowls. Add the toasted bread and cover each with 1/4 cup of the cheese mixture. Place the bowls on a baking sheet lined with tin foil. Place under the broiler until the cheese has melted and is slightly brown, about 2 minutes.