Heat a large, heavy bottomed pot over medium heat. Once the pot is hot, add the olive oil and swirl it around. Add the onions to the pot. Sauté and stir them occasionally until very soft and totally translucent, about 6-7 minutes. If they are beginning to brown, lower the heat.
Add the carrot and celery to the pot. Sauté with the onions, stirring occasionally, until celery is slightly softened around the edges, about 5 minutes. Season the vegetables with salt and pepper.
Add the garlic, chillies, thyme, and lemon zest to the pot and stir. Sauté until the garlic and lemon is super fragrant, about 1 minute.
Add the potatoes and chickpeas to the pot and season again with salt and pepper. Give everything a stir. Then add the vegetable stock to the pot. Give the soup another stir and place the lid on top.
Bring the soup to a boil and then lower the heat to a simmer. Set the lid askew on top of the pot so that some steam can escape. Keep simmering until the potatoes are very tender, about 15 minutes.
Carefully ladle half of the soup into the pitcher of an upright, vented blender. Blend this mixture on high until completely smooth. Pour the pureed portion of the soup back into the pot and stir. Add the baby spinach to the pot and stir again. Once the spinach is wilted, add the lemon juice and stir once more. Check the soup for seasoning and adjust if necessary (more lemon, salt, pepper, chili etc!).
Serve the soup hot with drizzles of olive oil and extra black pepper on top.