In a medium heavy saucepan, cook mushrooms, onions, and thyme in the chicken stock until tender. Set aside.
In another large saucepan, melt butter and whisk in flour until a roux forms. Add salt,pepper, half and half; Mix well. Then and the mushrooms, onions and thyme. Stirring constantly, bring soup to a boil and cook until thickened. Add sherry last and season to taste if you need.