Saute onion for 5 minutes in butter in a 4 quart saucepan. Add tomatoes, salt, pepper, baking soda and sage. Saute for 10 - 12 minutes or until thickened and pasty. Remove from heat and stir in cream. Taste for seasoning. Return to heat and heat through.

Serve in heated cup or refrigerate and serve chilled with a dollop of sour cream and chopped parsley.