Heat the butter in a large dutch oven over medium heat. Add onion and 1/2 tsp kosher salt. Cook until softened and translucent (not browned), about 6 minutes.
Stir in the flour and cook one minute, stirring constantly, then gradually whisk in milk.
Add baking soda, tomatoes, sugar and 1 tsp more salt. Simmer 10 minutes, then blend with an immersion or regular blender (see note below).
Add more salt by pinches, tasting as you go until soup tastes vibrant and well-seasoned (see note below).
Notes
On seasoning to taste, I use about 2 tsp kosher salt, but it really depends if the butter and tomatoes had salt.To blend the soup in a regular blender, you must work in small batches as hot liquids will come shooting out if you overfill. Do a few ladlefuls at a time, pour the pureed soup into another pot, and continue until all the soup is pureed.