In large, wide saucepan, add olive oil and sauté onion, carrots, and celery over medium heat for about 2 minutes, seasoning with 1/4 tsp salt. Add garlic and sauté for further 3 minutes, until onions are translucent. Add rinsed chickpeas, reserved chickpea liquid, and vegetable stock. Simmer gently on medium-low heat, or until vegetables are tender. Add spinach, lemon juice, and tarragon; stir to incorporate, allowing heat to wilt spinach. Test for seasoning and add remaining salt, if needed, and pepper.