In a large saucepan, heat the butter or margarine and the vegetable oil over medium heat. Add the onion and saut* until softened and golden, about 10 minutes. Add the tomatoes (undrained), the thyme, basil, and black pepper. Bring the mixture to a simmer,
In a small bowl, combine the flour and broth, whisking till 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 burn