In a saucepan over high heat, sauté the onions and mushrooms in the butter until the liquid has evaporated and the vegetables are golden brown, about 15 to 20 minutes. Season with salt and pepper.
Sprinkle in the flour and blend well. Deglaze with the chicken broth. Bring to a boil, stirring frequently. Simmer for 5 minutes. Add the oysters and their juices. Remove from the heat. Cover and set aside.
With the rack in the highest position, preheat the broiler.
Ladle the soup into 6 ovenproof bowls. Top with a slice of bread. Sprinkle with cheese and brown under the broiler.
You can also broil the bread sprinkled with cheese then place it on the hot soup.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.