In a large saucepan, heat butter and oil over medium heat until butter is melted. Add onion; cook and stir until tender, 5-7 minutes. Add garlic; cook 1 minute longer. Stir in tomatoes, chicken stock, tomato paste, basil, salt, sugar, thyme and pepper. Bring to a boil. Reduce heat; simmer, uncovered, to let flavors blend, 20-25 minutes. , Remove pan from heat. Using a blender, puree soup smooth. Return to pan. Slowly stir in cream. Cook and stir over low heat until heated through. If desired, garnish with basil and additional cream.