In a large saucepan, cook onion in butter until soft. Stir in flour, stirring constantly until bubbly. Stir in clams with liquid and clam juice. Continue to cook until mixture thickens and boils 1 minute. Cover and simmer for 15 minutes. Blend in cream, tomato paste and lemon juice. Heat slowly until hot. A 6 qt pan will work but only has a couple inches at the top.