Gather all ingredients.
Simmer mushrooms, stock, onion, and thyme in a large heavy saucepan until vegetables are tender, 10 to 15 minutes.
Carefully transfer the hot mixture to a blender or food processor. Cover and hold lid down with a potholder; pulse until creamy but still with some chunks of vegetable.
Melt butter in the same saucepan. Whisk in flour until smooth. Whisk in salt and pepper. Slowly whisk in half-and-half and mushroom mixture.
Bring soup to a boil and cook, stirring constantly, until thickened.
Stir in sherry. Taste and season with more salt and pepper if needed.
