In a large kettle, saute garlic in 2 tablespoons butter.
Add onions, celery and green pepper; saute until tender.
Add mushrooms; saute for 5 minutes. Stir in broth and simmer for 20 minutes. In a saucepan, melt remaining butter; stir in flour until smooth. Gradually stir in milk. Cook and stir over low heat until mixture comes to a boil; boil for 2 minutes.