Place chicken in a processor or blender. Add lemon juice and blend or process at low speed. Add onion, 1 tablespoon parsley, celery, yogurt, milk and process until a thick cream is obtained. Season with salt and pepper.
Remove from blender or processor and transfer cream to a bowl. Add mushrooms, reserving 10 slices for decoration. Correct seasoning.
Pour mixture into a pate mold, lightly greased. Refrigerate for 3 hours.
Unmold and cover the top with chopped parsley and mushroom slices.