Melt the butter in a soup-pot. Add the onion and cook over medium heat, stirring, until the onion is soft but not browned. Sprinkle the flour over the butter mixture and continue to cook for an additional 2 minutes. Slowly add the milk, bay leaf, sugar, and salt and continue to cook and stir until slightly thickened. Stir the baking soda into the tomatoes. Add the tomatoes to the milk, and bring just to a simmer. Remove from the heat and put through a strainer. Taste and correct the seasoning. Reheat before serving.