Combine butter and oil in soup pot; add onions; saute over low heat, sprinkling with thyme, basil, salt and pepper; stir occasionally. When onions are tender, stir in tomatoes and tomato paste; simmer about 18 minutes. Blend flour with equal amount of chicken broth; add to tomato mixture and mix well. Slowly add remaining chicken broth, stirring well. Simmer covered, over low heat, for about 30 minutes. Stir occasionally to prevent soup from sticking to bottom of pan. Put soup through blender in small batches. Reheat puree and stir in sugar, cream and milk. Simmer a few minutes; add remaining butter just before serving.