Place a large saucepot over medium high heat. Add the butter, mushrooms and onions. Sauté until lightly browned. The mushrooms will initially release a lot of moisture, which will eventually evaporate allowing them to brown. Season with salt and pepper.
Add the Shiraz and bring to a simmer, reducing it by half. Add the chicken broth and cream, stir well and bring to a simmer. Add the thyme and continue cooking for a few minutes. Meanwhile stir the cornstarch into a ¼-cup of water. Add the mixture slowly to the simmering soup, stirring until it returns to the simmer and thickens slightly. If you prefer your soup a bit thicker feel free to add more cornstarch. Taste and add salt and pepper as desired.
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