Heat a large saucepan over high heat. When the pot is hot, spray it with cooking spray. Add the mushrooms, onion and garlic. Saute until the mushrooms have released most of their liquid and are starting to become tender, about 5 minutes.
Season the mushrooms with salt and pepper to taste. Add the chicken broth and evaporated skim milk. Cover, and bring the soup to a boil. Reduce heat and simmer until the mushrooms are completely tender, about 8 minutes.
Working in two batches, puree the soup in a blender until smooth. Return the soup to the pot to reheat. Season with salt and pepper to taste, and serve.