Trim tough stem ends of mushrooms; remove stems; set aside. With knife, slice mushroom caps thinly.
In 4-quart saucepan over medium-high heat, in hot butter, cook sliced mushrooms and lemon juice until mushrooms are just tender, stirring.Reduceheat to medium-low; with slotted spoon, remove mushrooms to bowl.
In saucepan in remaining butter, cook onion and stems; cook until onion is tender. Stir in flour until blended; cook 1 minute, stirring the mixture constantly. Gradually stir in chicken stock; cook, stirring constantly, until mixture is thickened.
Into blender container, ladle half of mixture; cover and at high speed, blend until smooth. Repeat with other half. Return mixture to saucepan; stir in salt, pepper, cream and mushroom slices; reheat just until soup is boiling.