Clean mushrooms, discard any hard steams and chop into little pieces
In a large heavy bottomed pot or Dutch oven, melt butter, add onions and cook until soft 5-8 minutes Add mushrooms, salt, pepper, cover over medium heat, stirring often until mushroom release their liquid, about 10 minutes
Add flour and stir to coat the mixture, cook for 1 minute. Add sherry and cook for an additional minute
Stir in chicken broth, thyme leaves, bring to a boil
Reduce heat to maintain a simmer and cook for 20 minutes. Stirring occasionally
Remove from heat. Transfer 1 ½ cups of soup to a blender or food processor and puree until smooth
Stir pureed soup back into the pot
Stir in heavy cream and season with salt and pepper to taste
Gently warm over low heat, if needed, and serve.
