In a medium saucepan, melt 2 tablespoons of the butter. Add the chopped onion, carrot, celery and garlic, cover and cook over moderately high heat, stirring occasionally, until the vegetables are just beginning to brown, about 5 minutes. Sprinkle the flour over the vegetables and stir over low heat for 1 minute, or until the flour is fully incorporated. Add the chicken stock, tomatoes, tomato paste and sugar and bring to a boil. Cover partially and cook the soup over moderate heat, stirring occasionally, until the vegetables are tender, 15 minutes.