In a medium saucepan (3-4 quart) on medium heat saute the onion and garlic with butter, cook for 1 minute.
Add in the mushrooms and chicken broth.
Bring to a simmer.
Mix cornstarch with milk, and slowly pour into the simmering broth.
When the soup comes to a simmer again, turn the soup off.
Add salt and pepper to taste.
Puree the soup with an immersion blender (or food processor or blender)
and serve.
Top with croutons, grated cheese (Romano cheese) green onions, or bacon.