Heat 2 tablespoons butter in a pot over medium-high heat. When the butter has melted, add a third of the mushrooms to the pot. Cook, stirring occasionally, until dark golden brown, about 5 minutes. Season with salt and pepper, then remove them from the pot and reserve for serving.
In the same pot, melt the remaining 2 tablespoons butter. Reduce the heat to medium and, when the butter has melted, add the rest of the mushrooms, along with the garlic, celery, thyme and onion. Cook, stirring frequently, until the vegetables are soft, 5 to 6 minutes. Sprinkle the vegetables with the flour and stir to combine. Season with salt and pepper, then cook, stirring constantly, for 3 minutes, allowing the flour to cook.
Slowly pour in the sherry, stirring all the time, then slowly add the stock. Keep stirring to reduce the likelihood of lumps. Bring to a simmer and cook, stirring occasionally, until slightly reduced and thickened, about 20 minutes.
Using an immersion blender, puree until the mushrooms are mostly broken up into small barley-sized bits. (NOTE: You may use a blender, though allow the soup to cool until warm before pureeing.) Add the cream and heat until the pot is simmering. Add the balsamic, then taste and adjust the seasonings.
Serve warm in bowls with the reserved mushrooms on top. Garnish with parsley and crusty chunks of warm bread.
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