Place a large stockpot over medium high heat. Add the butter, mushrooms and onions. Season with salt and pepper. Sauté until liquid from mushrooms had evaporated and mushrooms had turned lightly browned, about 30 minutes.

Pour in the red wine and bring to a simmer. Continue cooking until the wine has reduced by about two-thirds, about 10 minutes.

Add the chicken broth, heavy cream and fresh thyme. Stir well and bring the soup to a simmer.

Meanwhile dissolve the cornstarch in the water. Add this mixture slowly to the simmering soup, stirring until it returns to a simmer and thickens slightly. If you prefer your soup a bit thicker, feel free to add more cornstarch. Taste and add more salt and pepper, if desired. Serve immediately.