In a large saucepan, heat the butter and vegetable oil over medium heat. Add the onion and sauté until softened and golden, about 10 minutes., Add the tomatoes, basil, thyme, and black pepper. Bring the mixture to a simmer, and cook for 10 minutes., In a small bowl, combine the flour and broth, whisking until smooth, and add this mixture to the soup, stirring constantly., Cover and simmer slowly for 25 minutes, stirring occasionally., If you've used diced tomatoes, purée the soup in a blender or food processor, or use a hand blender. If you've used tomato purée, there's no need to blend; the onion bits will give the soup a bit of body., Return the soup to the stove, and set it on a burner over low-to-medium heat., Stir in the baking soda (the soup will foam up briefly; don't worry, but be sure it's in a big enough pot), the sugar, milk, and salt. Heat to a bare simmer, stirring constantly. Serve hot.