Heat the oil in a large saucepan or stockpot, add zucchini and onion. Saute for 5 minutes or until onions are translucent and zucchini is crisp tender. Then add stock, bring to a boil, reduce heat and partially cover and cook for 25 minutes. Add basil during the last 5 minutes of cooking. Mix the butter and flour together into a paste. Remove 1 cup of simmering stock and whisk in butter mixture until smooth. Add back into soup, stir until thickened. Remove soup from heat to a blender and puree until smooth. Taste and season with salt and pepper. Add lemon juice and serve with a dollop of sour cream or yogurt.