Trim tough stem ends of the mushrooms off and discard; remove remaining stems; set aside. 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 often.
Reduce heat to medium low; with slotted spoon, remove mushrooms from bowl. In saucepan add in remaing butter, onions and mushroom stems; cook until onion is tender.
Stir in flour until blended; cook one minute stirring the mixture constantly. It will become a thick paste.
Gradually stir in the chicken broth; cook, stirring constantly until mixture is thickened.
Return all but a small handful of mushroom slices to the pot. If you do not have a stick mixer, you can use a conventional blender being careful to puree the mixture in parts so as not to overflow the blender.
Once the mixture in a saucepan has been pureed; stir the salt, pepper, and the handful of mushroom slices.
In a separate bowl combine a ladle of the warm soup and the whipping cream to acclimatize the cream so it won't clot. Then carefully stir in the creamy mixture in to the soup pot. Add pepper to taste.