Melt the butter in a soup pot. Add the onion and cook over medium, stirring, until onion is softened but not browned. Sprinkle the flour over the butter mixture and continue to stir and cook for about 1 minute. Slowly add milk, broth, bay leaf, sugar and salt and cook and stir until slightly thickened. Stir in baking soda. Add tomatoes and bring just to a simmer. Remove from heat and put in a blender in batches to puree. taste and correct seasonings. Reheat before serving.
Note: use small batches in the blender (1/2 full at a time). Blending hot liquids can be very dangerous.