Sauté shallots and garlic in a chunk of butter for about 3 minutes. Do not brown. Add flour. Stir-cook another minute.
Add tomatoes and soda. Stir-cook until hot. Add cream, milk, salt, sugar, and pepper. Bring just to a boil. Grind through a food mill and into a clean pan. Heat to hot. Remove, and swirl-in a large piece of butter. Taste and adjust seasonings