In a large pot, melt 2 tbsp of butter over medium high heat. Add the mushrooms. Cook until dark golden brown season with salt and pepper.
Once the mushrooms are dark brown remove them from the pot and place them on a plate, setting aside for later.
Add the remaining 2 tbsp of butter and add onion, garlic, and celery. cook, stirring often, about 3-5 minutes until vegetables soften.
Add the mushrooms back into the pot, thyme, and bit more salt and pepper
Sprinkle the flour over the vegetables
Cook for about 3 minutes
Pour in the sherry and stir, then add the both broths
Let the soup cook for about 20 minutes
Using an immersion blender puree the mixture together to the consistency you prefer. I like a little bit of "chunk" left in it.
Add the cream and balsamic vinegar and any salt and pepper you may think it needs, tasting the soup to make sure the seasoning is ok.Happy Eating!!