Trim tough stem ends of mushrooms; remove stems; set stems aside. Slice mushroom caps thinly.
In 4-quart saucepan over med-high heat, in hot butter, cook sliced mushrooms and lemon juice until mushrooms are just tender, stirring occasionally.
Reduce heat to med-low; with slotted spoon, remove mushrooms to bowl. In butter remaining in saucepan, cook onions and stems; cook until onion is tender.
Stir in flour until blended; cook 1 minute. Gradually stir in water and bouillon; cook, stirring constantly, until mixture thickens and boils.
Spoon one-half of mushroom mixture into blender; cover (with center part of blender cover removed) and blend at low speed until smooth. Pour mixture into bowl. Repeat with remaining mushroom mixture.
Return mixture to saucepan; stir in salt, pepper, cream and mushroom slices; reheat just until soup is boiling. 4-6 servings.