Heat the oil in a frying pan over medium heat. Cook and stir the zucchini and garlic in the frying pan until lightly browned. Remove from heat, and set aside.
In a blender or food processor, purée the tomatoes, onion and chilli, leaving a few small chunks.
In a large pot, mix the tomato purée and vegetable stock. Season with tarragon, dill, salt and pepper. Bring to the boil, reduce heat to low, and mix in zucchini and garlic. Cover, and simmer 45 minutes.