Place sliced onions in medium bowl and add cold water to cover and cider vinegar. Let soak at least 15 minutes while preparing remaining ingredients. Drain, use towel to squeeze out excess moisture. Roughly chop; a food processor saves time.
Place a 6-quart stock pot over medium heat. Add butter or margarine and melt. Add onions and garlic; saute until onion is transparent, approximately 4 minutes. Add celery; saute until celery begins to soften, approximately 4 minutes.
Add tomatoes, broth, and tomato paste; stir to blend well. Cover and bring to a boil over high heat. Add rice, cover, and reduce heat to simmer. Cook until rice is tender, approximately 20 to 25 minutes.
Remove from heat, cool slightly. Use an immersion blender or blender to puree until very smooth.
Add salt and pepper to taste. If too thick, adjust consistency using additional broth. If planning freeze a portion, set it aside before adding cream. Use a whisk to slowly add correct portion of cream to remaining soup. Reheat, if necessary. Garnish with chopped fresh parsley and serve hot.