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1
Heat 2 tablespoons butter in a pot over medium-high heat. When melted, add 1/3 of the mushrooms. Cook them until dark golden brown, stirring occasionally, about 5 minutes. Season with salt and pepper, then remove them from the pot and set them aside.
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2
In the same pot, melt the remaining 2 tablespoons butter. Reduce the heat to medium and add the rest of the mushrooms, along with the onion, celery, and thyme. Cook, stirring frequently, until vegetables are soft, about 5 to 6 minutes.
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3
Sprinkle the vegetables with the flour and stir to combine. Season with salt and pepper and stir constantly for 3 minutes, allowing the flour to cook. Slowly add the wine or sherry, stirring while you add. Allow it to heat and bubble up for a couple of minutes, then slowly pour in the stock. Keep stirring to reduce the likelihood of lumps.
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4
Bring to a simmer and cook for 20 minutes, stirring occasionally. Remove it from the heat and let it cool for 20 minutes. Transfer to a blender (make sure it is just warm or cool at this stage-blending hot soup is dangerous!) and puree until mushrooms are mostly broken up into small barley-sized bits. (NOTE: You may use an immersion blender in the pot if you have one, and you don't have to let the soup cool first.) Transfer the soup back to the pot, add the cream, and heat it to a simmer. Add balsamic, then taste and adjust seasonings.
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5
Serve warm with reserved mushrooms on top and crusty chunks of warm bread.
