In 5 quart Dutch oven, combine bacon, mushrooms, onion and garlic. Cook and stir over medium-high heat until bacon is crisp and vegetables are tender; drain.
Stir in broth and spinach; simmer 5 minutes.
In small bowl, combine milk and flour; beat with wire whisk until well blended. Stir flour mixture into soup. Simmer until mixture boils and thickens, stirring constantly.