Melt butter in a Dutch oven or soup pot over medium high heat. Add mushrooms and onions and saute for 4 to 5 minutes, or until tender.
Whisk together flour, salt, pepper, thyme, garlic powder and 2 cups of the chicken broth in a medium bowl until smooth. Pour into mushroom mixture and stir to combine.
Add remaining broth and bay leaf and stir to combine. Bring to a boil. Cook, stirring constantly for about 1 to 2 minutes, or until thickened.
Reduce heat to maintain a gentle simmer. Add half and half and stir to combine. Simmer (uncovered) for 10 minutes, stirring occasionally. Season with additional salt and pepper, if desired. Remove and discard bay leaf before serving.