Heat the butter in a large pot over medium heat. Add onion and cook until softened and translucent (not browned), about 6 minutes. Stir in flour, cook one minute, then gradually whisk in milk. Add baking soda, tomatoes and sugar. Simmer 10 minutes, then blend (either with an immersion blender, or in small batches in a regular blender - careful, hot liquids will explode out if you overfill!). Add salt by pinches, tasting as you go until soup tastes vibrant and well-seasoned.