In a large saucepan, saute the mushrooms, onion and garlic in oil until tender; remove with a slotted spoon an set aside. In the same pan, combine flour and broth until smooth. Bring to a boil; cook and stir for 1-2 minutes or until thickened.
Return mushroom mixture to saucepan. Add the tomatoes, basil, sugar, salt and pepper. Cook over medium heat for 5 minutes or until heated through.