1In a large saucepan, combine the mushrooms, onion and chicken broth. Bring to a boil, then simmer covered for 15 minutes over low heat.
2Stir together the melted butter and flour to make a paste. Stir the paste into the pan with the vegetables. Increase the heat to medium, and gradually stir in the milk. Continue stirring constantly.
3When the mixture thickens and begins to boil, stir in the cream. Cook over low heat without boiling for about 10 minutes, or until the mushrooms are tender. Season with salt and pepper.
4Ladle the soup into bowls, and trim pieces of toast to fit the bowls. Butter the toast, and place on top of the soup. Sprinkle the cheese over the bread and serve.