Melt the margarine in a large Dutch oven over medium-high heat. Add the mushrooms, and sauté for 3 minutes. Stir in the flour. Gradually add the milk, stirring with a whisk. Add the potato, thyme, salt, pepper, and corn, and bring to a boil. Stir in the shredded chicken and 2 tablespoons green onions; cover, reduce heat, and simmer for 15 minutes. Ladle the soup into bowls, and sprinkle with 1 tablespoon green onions.