Melt margarine in a heavy-based pan and cook green onion, garlic and thyme for 1 minute, or until the garlic is golden (stir during cooking). Add mushrooms and cook 3-4 minutes, or until mushrooms just soften. Add flour and cook, stirring, for 1 minute. Remove from heat and add broth, stirring continuously. Return to heat and bring to a boil while stirring. Reduce heat and simmer gently for 2 minutes, stirring occasionally. Whisk the cream into the soup. Do not allow the soup to boil. Ladle into bowls and garnish with chives.