In a large saucepan heat olive oil over medium-high heat. Add onion and garlic and cook for approximately 5 minutes or until the onion is tender. Add flour and cook for 2 to 3 minutes, stirring constantly. Pour in chicken stock and bring to a light boil, stirring until thickened. Add tomatoes, thyme, and basil. Reduce the heat to a simmer and cook for 10 minutes. Stir in additional chicken stock if desired. Remove from heat and allow to cool slightly.
Transfer soup in batches to a blender or food processor. Pulse until smooth. Pour soup into a large saucepan and season with kosher salt and white pepper to taste. Keep warm on low heat until ready to serve.
Ladle soup into bowls. Garnish with lemon zest and drizzle with olive oil.