In a large bowl, toss the sliced mushrooms, chopped onion, and minced garlic. Transfer to the slow cooker.
Pour in the vegetable or chicken broth. Add dried thyme and rosemary. Stir to combine.
Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, until mushrooms are very tender.
In a separate small bowl, whisk together the melted butter and flour until a smooth paste (roux) forms.
Once the soup is cooked, stir the roux into the hot soup. Whisk well to ensure no lumps. Continue to stir occasionally over the next 15-20 minutes as the soup thickens.
Stir in the heavy cream. Season with salt and black pepper to taste.
For a smoother soup, you can use an immersion blender to blend a portion or all of the soup to your desired consistency. (I recommend blending about half for texture).
Serve hot, garnished with fresh parsley or chives if desired.
Notes
Using a mix of mushrooms will provide a deeper, more complex flavor.
For a vegetarian option, ensure you use vegetable broth.
The roux can be made ahead of time and stored in the refrigerator.
Adjust the amount of cream to achieve your desired richness.
Nutrition
Serving Size:1.5 cups
Calories:Approximately 350-450 (will vary based on broth and cream fat content)