In a skillet, heat the butter over medium heat until melted. Add the mushrooms and thyme and sauté for about 5 minutes. Transfer the mushrooms to a 4- to 6-quart slow cooker.

Add the remaining ingredients except the heavy cream. (Alternatively, do not brown the mushrooms and add all the ingredients except heavy cream to the slow cooker.)

Cover and cook on LOW for 8 hours.

Stir in the heavy cream. Cover and cook on LOW for about 30 minutes.